• Add basic pyproject.toml with build-system information
• Drop versioneer in favor of versioningit
• Delete versioneer install module and remove its setup.cfg config
• Replace src/streamlink/_version.py and load streamlink's version
  string via versioningit. The module's __version__ export will get
  replaced by a static string on build-time when creating sdists/wheels,
  which results in the old versioneer behavior with support for
  editable installs.
• Make versioningit use the same version format as versioneer
• Update MANIFEST.in and .coveragerc

Resolves #4321

This needs to be thoroughly reviewed before it can be merged. I'm opening this as a draft for now.

I've set the required setuptools version to >=45, which is the one which breaks compatibility with python 2. A higher version could be set here, but it's probably not necessary. At least version 42 is necessary for PEP 517/518.

The version string format generated by versioningit remains the same, so it'll be one of these, depending on the current state

• {version}
• {version}+{distance}.{vcs}{rev}
• {version}+{distance}.{vcs}{rev}.dirty

Editable installs via pip install -e . should be working. Please see src/streamlink/_version.py, which loads the current version from versioningit unless the file's __version__ export was replaced during build-time (sdist/bdist) with a static version string.

The next step would be moving the project metadata from setup.py to pyproject.toml, and dependencies from {dev,docs}-requirements.txt to the project.optional-dependencies table of pyproject.toml. Maybe also properly defining dev- and doc-dependency version ranges would be a wise idea, at least minimum versions. I'd also love to have a dependency lockfile here, but that can be done later.