chore: Bump version to 0.3.0 and derive __version__ from package metadata

[shenxianpeng]

Summary

Prepares the 0.3.0 release, and removes the hand-maintained version constant
that made the last release ship a wrong version string.

Two commits

1. chore: Bump version from 0.2.0 to 0.3.0

File	Before	After
Cargo.toml	0.2.0	0.3.0
Cargo.lock (own package entry)	0.2.0	0.3.0
python/gitoxide/__init__.py	0.1.0	0.3.0

Note the third row: __version__ was still 0.1.0. It was missed during the
0.2.0 release, so the published 0.2.0 package reports 0.1.0 at runtime.

2. chore: derive __version__ from installed package metadata

Rather than leave a constant that has to be remembered on every release,
__version__ is now read from the installed distribution metadata, which
maturin populates from the Cargo.toml version at build time:

try:
    __version__ = _version("gitoxide")
except PackageNotFoundError:  # pragma: no cover - source tree without an install
    __version__ = "0.0.0+unknown"

Cargo.toml becomes the single place a release version is written —
pyproject.toml already sources it from there. A test_version case guards
against the metadata lookup silently degrading to the sentinel value.

Verification

Built and installed into a clean venv via pip install . (the same PEP 517
path CI uses), then imported from outside the source tree:

gitoxide.__version__      = 0.3.0
包元数据 version          = 0.3.0

pytest -q → 16 passed. cargo metadata --locked confirms Cargo.lock
stays in sync with Cargo.toml.

Release ordering

publish.yml triggers on v* tags and builds the version from Cargo.toml,
so this must merge before the v0.3.0 tag is pushed — tagging first is
what produced the earlier mismatch.