Remove Python 2 compatibility and add mypy type checking

[Copilot]

Drops Python 2 support, sets minimum version to Python 3.8, and introduces gradual type checking with mypy.

Compatibility Removal

• Simplified Python 3 implementations: Removed conditional logic from intbytes.py, encoding/bytes32.py, ecdsa/intstream.py, ecdsa/rfc6979.py that provided Python 2 fallbacks for int.to_bytes(), int.from_bytes(), and int.bit_length()
• Updated imports: Removed from __future__ import print_function from cmds modules; replaced urllib2/urllib compatibility layer with direct Python 3 imports
• Fixed exceptions: Changed raise NotImplemented() to raise NotImplementedError() in base classes (TxIn, TxOut, Tx, SolutionChecker)

Type Annotations

Added type hints to core utility modules with Literal types for strict parameter validation:

# Before
def to_bytes(v, length, byteorder="big"):
    return v.to_bytes(length, byteorder=byteorder)

# After  
def to_bytes(v: int, length: int, byteorder: Literal["big", "little"] = "big") -> bytes:
    return v.to_bytes(length, byteorder=byteorder)

Annotated modules: intbytes, encoding/bytes32, encoding/hexbytes, ecdsa/intstream, ecdsa/rfc6979, merkle, bloomfilter

Configuration

• setup.py: Added python_requires='>=3.8', classifiers for Python 3.8-3.13
• tox.ini: Test environments now py38-py313 with coverage
• mypy.ini: Gradual typing config (python_version=3.8, ignore_missing_imports=True, disallow_untyped_defs=False)
• CI: Removed test-py27.yml, added mypy.yml workflow, updated test matrix to 3.8-3.13
• Distribution: Added py.typed marker for PEP 561 compliance

Development

Created requirements-dev.txt with pytest, coverage, mypy dependencies.

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Original prompt

Summary

Remove all Python 2 compatibility code from the repository richardkiss/pycoin (branch: main) and add mypy static type checking to the codebase and CI. The PR should keep runtime behavior unchanged for Python 3.x users; it should simply stop maintaining Python 2 compatibility shims and add gradual type annotations and a mypy config and CI job. The changes should be small, reversible, and well-documented in the PR description.

Goals

1. Remove Python 2 compatibility vestiges across the codebase (no behavioral changes for Python 3):

   • Remove references to and usage of six, future, and any custom compatibility modules.
   • Replace Python-2-specific aliases/usages: xrange -> range, iteritems()/itervalues()/iterkeys() -> items()/values()/keys(), long -> int, basestring -> str, unicode -> str, dict.iteritems -> dict.items, dict.iterkeys -> dict.keys, dict.itervalues -> dict.values.
   • Remove future imports that are unnecessary on Python 3 (except those that are still relevant; prefer to remove ones related to python2 compatibility like unicode_literals if present and adjust code accordingly).
   • Remove try/except blocks that provide alternate code paths for Python 2 (e.g., except NameError: define bytes/str shims) and instead rely on Python 3 builtins.
   • Update setup.py / setup.cfg / pyproject.toml:
     • Remove Python 2 trove classifiers.
     • Update python_requires to a sensible Python 3 only range (e.g., >=3.8) — choose 3.8 to 3.11 compatibility unless tests suggest otherwise.
     • Remove or update dependency pins that were only required for Python 2 compatibility (e.g., six) and move typing-only packages to dev dependencies.
   • Remove any code specifically marked as Python2-only or guarded by if sys.version_info[0] < 3 or similar.

2. Add mypy to the project and progressively annotate the code.

   • Add a mypy configuration file (mypy.ini or setup.cfg [mypy]) at the repo root with sensible defaults for gradual typing:
     • strict optional False
     • disallow_untyped_defs False initially
     • plugins none
     • ignore_missing_imports True
     • files: src or top-level package(s) as appropriate
   • Add type annotations to all public functions and methods across the codebase. Prioritize public API surfaces, modules under the top-level package, and modules most used by tests.
   • Add typing imports (typing: Any, Optional, Union, Dict, List, Tuple, Iterable, Iterator, Callable) and use Python 3.8+ annotations (from future import annotations if needed), but since we're removing Python 2 support, we can safely use postponed evaluation of annotations by adding "from future import annotations" at top of modules where forward references are needed and Python 3.7+ supported. Prefer using current annotations (PEP 563) if target python allows.
   • Where complete typing is impractical in the timeframe, add precise typed stubs or use "# type: ignore[assignment]" or "-> Any" with TODO comments to follow up.
   • Add a py.typed marker file at the package root if the package should be typed for consumers.

3. Add CI job and developer tooling to run mypy.

   • Create a GitHub Actions workflow .github/workflows/mypy.yml that runs on push and PR to main and runs mypy with the repository's configuration. Also run mypy in a reasonable Python matrix (e.g., 3.8, 3.10, 3.11) but at minimum 3.8+.
   • Add mypy and typing-related packages to dev-requirements or tests/requirements (e.g., mypy>=1.0, types-requests if necessary), and update any existing CI workflow to include type-check step.
   • Optionally add a pre-commit hook entry for mypy in .pre-commit-config.yaml if present.

4. Run quick automated code modernizations where safe.

   • Replace "%"-style formatting in places flagged as Python2-compat if appropriate with f-strings for readability.
   • Replace explicit list(dict.keys()) usages where unnecessary.
   • Convert textual bytes/str conversions to Python 3 idioms.
   • Ensure explicit use of pathlib where previously os.path-based shims existed if small and safe.
   • All automated changes must preserve behavior and tests should pass.

5. Testing and validation

   • Run the existing test suite (if present) and make minimal test fixes required by compatibility removals.
   • Ensure no tests are skipped because of removed Python 2 compatibility code.
   • Ensure linting and style do not degrade (optional: run flake8/ruff if available in CI).

Scope and Files to Change

• Apply changes across the entire repository tree under the main branch (richardkiss/pycoin main).
• Key targets (where to look and what to change):
  • setup.py, setup.cfg, pyproject.toml, requirements*.txt, dev-requirements
  • top-level package directory (likely pycoin or package name) — annotate public functions/methods
  • modules that import six, future, or define compatibility helpers
  • init.py files to add py.typed if appropriate
  • tests/ to update type hints where necessary for mypy
  • .github...

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