chore(deps): bump pytest, black, mypy, hypothesis with Py39 compat

Update dev dependencies to their latest versions while maintaining
Python 3.9 support via environment markers:

- pytest: 8.4.* -> 9.1.* (Py39 stays on 8.4.*)
- black: 25.* -> 26.* (Py39 stays on 25.*)
- mypy: 1.19.* -> 2.2.* (Py39 stays on 1.19.*)
- hypothesis: ~=6.0 -> ~=6.156 (Py39 gets ~=6.141)

All four packages dropped Python 3.9 support in their latest major
releases. This uses the same conditional pattern as openjd-cli.

Supersedes #300, #301, #302, #303.

Signed-off-by: Jericho Tolentino <68654047+jericht@users.noreply.github.com>

Author: jericht

requirements-testing.txt

@@ -1,15 +1,19 @@
 coverage[toml] == 7.*
-pytest == 8.4.*
+pytest == 9.1.*; python_version >= "3.10"
+pytest == 8.4.*; python_version < "3.10"
 pytest-cov == 7.1.*
 pytest-timeout == 2.4.*
 pytest-xdist == 3.8.*
 types-PyYAML ~= 6.0
-black == 25.*
+black == 26.*; python_version >= "3.10"
+black == 25.*; python_version < "3.10"
 ruff == 0.15.*
-mypy == 1.19.*
+mypy == 2.2.*; python_version >= "3.10"
+mypy == 1.19.*; python_version < "3.10"
 # pydantic: required by the legacy model_v0 (Pydantic) test suite,
 # which lives alongside the new Rust-backed model_v1 implementation.
 pydantic >= 2.10, < 3
 # hypothesis: used by the opt-in fuzz test suites in test/openjd/expr
 # and test/openjd/model-v*. conftest.py imports it unconditionally.
-hypothesis ~= 6.0
+hypothesis ~= 6.156; python_version >= "3.10"
+hypothesis ~= 6.141; python_version < "3.10"