build: remove versioningit build-req from sdist

[bastimeyer]

• Replace tool.versioningit.onbuild hook with a custom implementation which replaces the entire streamlink._version module (similar to before) and which additionally removes versioningit from the build-system.requires field in pyproject.toml and which sets a static version string in setup.py
• Rewrite streamlink._version module
• Add and update comments
• Update docs
• Add tests

---

Follow-up of #5625

sdist and bdist build just fine...

$ python -m build --sdist --wheel
...

pyproject.toml and setup.py changes in sdist:

$ head -n8 pyproject.toml 
[build-system]
requires = [
  "setuptools >=64",
  "wheel",
  # The versioningit build-requirement gets removed from the source distribution,
  # as the version string is already built into it (see the onbuild versioningit hook):
  "versioningit >=2.0.0, <3",  # disabled in sdist
]

$ bsdtar -xzOf dist/streamlink-6.2.1+41.g3494ec48.tar.gz streamlink-6.2.1+41.g3494ec48/pyproject.toml | head -n8
[build-system]
requires = [
  "setuptools >=64",
  "wheel",
  # The versioningit build-requirement gets removed from the source distribution,
  # as the version string is already built into it (see the onbuild versioningit hook):
  # "versioningit >=2.0.0, <3",
]

$ tail -n6 setup.py 
    setup(
        cmdclass=get_cmdclasses(),
        entry_points=entry_points,
        data_files=data_files,
        # version="",  # static version string template, uncommented and substituted by versioningit's onbuild hook
    )

$ bsdtar -xzOf dist/streamlink-6.2.1+41.g3494ec48.tar.gz streamlink-6.2.1+41.g3494ec48/setup.py | tail -n6
    setup(
        cmdclass=get_cmdclasses(),
        entry_points=entry_points,
        data_files=data_files,
        version="6.2.1+41.g3494ec48",
    )

streamlink._version module

$ tail -n1 src/streamlink/_version.py 
__version__ = _get_version()

$ bsdtar -xzOf dist/streamlink-6.2.1+41.g3494ec48tar.gz streamlink-6.2.1+41.g3494ec48/src/streamlink/_version.py
__version__ = "6.2.1+41.g3494ec48"

$ bsdtar -xzOf dist/streamlink-6.2.1+41.g3494ec48-py3-none-any.whl streamlink/_version.py
__version__ = "6.2.1+41.g3494ec48"

wheel metadata

$ bsdtar -xzOf dist/streamlink-6.2.1+41.g3494ec48-py3-none-any.whl streamlink-6.2.1+41.g3494ec48.dist-info/METADATA | grep -E '^Version: '
Version: 6.2.1+41.g3494ec48

---

Installing from the sdist ✔️

$ rm -rf /tmp/venv
$ python -m venv /tmp/venv
$ source /tmp/venv/bin/activate
$ pip install dist/streamlink-6.2.1+41.g3494ec48.tar.gz
...
$ streamlink -V
streamlink 6.2.1+41.g3494ec48
$ pip list | grep streamlink
streamlink         6.2.1+41.g3494ec48
$ python -c 'import importlib.metadata;print(importlib.metadata.version("streamlink"))'
6.2.1+41.g3494ec48

Installing from the wheel ✔️

$ rm -rf /tmp/venv
$ python -m venv /tmp/venv
$ source /tmp/venv/bin/activate
$ pip install dist/streamlink-6.2.1+41.g3494ec48-py3-none-any.whl
...
$ streamlink -V
streamlink 6.2.1+41.g3494ec48
$ pip list | grep streamlink
streamlink         6.2.1+41.g3494ec48
$ python -c 'import importlib.metadata;print(importlib.metadata.version("streamlink"))'
6.2.1+41.g3494ec48

Installing from git ✔️

$ rm -rf /tmp/venv
$ python -m venv /tmp/venv
$ source /tmp/venv/bin/activate
$ pip install git+file:///home/basti/repos/streamlink@build/remove-versioningit-from-sdist
...
$ streamlink -V
streamlink 6.2.1+41.g3494ec48
$ pip list | grep streamlink
streamlink         6.2.1+41.g3494ec48
$ python -c 'import importlib.metadata;print(importlib.metadata.version("streamlink"))'
6.2.1+41.g3494ec48

Installing in editable mode ✔️

$ rm -rf /tmp/venv
$ python -m venv /tmp/venv
$ source /tmp/venv/bin/activate
$ pip install -e . versioningit
...
$ streamlink -V
streamlink 6.2.1+41.g3494ec48
$ pip list | grep streamlink
streamlink         6.2.1+41.g3494ec48 /home/basti/repos/streamlink
$ python -c 'import importlib.metadata;print(importlib.metadata.version("streamlink"))'
6.2.1+41.g3494ec48

Building wheel from sdist ✔️

$ rm -rf /tmp/venv
$ python -m venv /tmp/venv
$ source /tmp/venv/bin/activate
$ bsdtar -xz -C /tmp/venv -f dist/streamlink-6.2.1+41.g3494ec48.tar.gz
...
$ cd /tmp/venv/streamlink-6.2.1+41.g3494ec48
$ python install build
...
$ python -m build --wheel
...
$ bsdtar -xzOf dist/streamlink-6.2.1+41.g3494ec48-py3-none-any.whl streamlink-6.2.1+41.g3494ec48.dist-info/METADATA | grep -E '^Version: '
Version: 6.2.1+41.g3494ec48
$ pip install dist/streamlink-6.2.1+41.g3494ec48-py3-none-any.whl 
...
$ streamlink -V
streamlink 6.2.1+41.g3494ec48
$ pip list | grep streamlink
streamlink         6.2.1+41.g3494ec48
$ python -c 'import importlib.metadata;print(importlib.metadata.version("streamlink"))'
6.2.1+41.g3494ec48

[bastimeyer]

And for good measure, I also tested building using the PKGBUILD from the Arch Linux repos...

---

My initial plan was to override the sdist and build_py setuptools cmdclasses, just like versioningit does, but this led to problems when building the wheel, as the version metadata gets read using the setuptools.finalize_distribution_options entry point, so this was a dead end.

The current solution should be good enough. I don't care that it modifies setup.py instead of only modifying pyproject.toml (added a comment about that), and I also don't care that it's a simple regex substitution instead of parsing and replacing the toml file, because otherwise, another build requirement (only when building from git though) would need to be added.