• move static package metadata to setup.cfg
• rename deprecated license_file field to license_files

Remove setuptools arguments:

• test_suite:
  Unneeded, as we disable the deprecated setuptools test command

Keep dynamic setuptools arguments:

• version/cmdclass:
  For writing static version info via versioneer
• install_requires:
  Some dependencies still depend on special environment variables
  instead of package "extra" dependency fields
• entry_points:
  The gui_scripts entry point should only be set when building wheels
  for Windows via --plat-name={win32,win-amd64,cygwin}
• data_files:
  All data files are optional and should not cause the build to fail if
  they are missing, eg. when building from sdist

• https://packaging.python.org/tutorials/packaging-projects/
• https://packaging.python.org/guides/distributing-packages-using-setuptools/
• https://setuptools.pypa.io/en/latest/userguide/declarative_config.html

Static package metadata could've also been moved into pyproject.toml (PEP 518 / PEP 621), but this would've changed how Streamlink gets built from source by tools like pip, and this causes issues with the integrated versioneer module that's required by setup.py.

TODO (in future PRs):

• Add the pyproject.toml file, at least with the build-system data, as it defines setuptools as build tool, which is currently only implied as a default.
  I had already added it but then ran into issues with installing in develop mode (pip install -e) because it couldn't import versioneer.
• Define optional dependencies as extras_require instead of using env vars in setup.py
• Remove the NO_DEPS env var, as it's unused

Some quick checks:

python setup.py --version

• outputs correct version (required by script/build-and-sign.sh)

python setup.py build

• writes built project files (into build/lib)
• writes streamlink/plugins/.removed (via options.package_data)
• bakes version info via versioneer

python setup.py sdist

• includes correct source tree
• includes correct metadata and entry_points in embedded egg-info
• includes streamlink/plugins/.removed (via MANIFEST.in)
• bakes version info via versioneer

python setup.py bdist_wheel

• includes built project files
• includes correct metadata and entry_points (Windows wheels also have the gui_scripts entry point)
• includes streamlink/plugins/.removed (via options.package_data)
• includes optional data files (shell completions and man pages)
• bakes version info via versioneer

python setup.py install

• writes built project files
• writes correct metadata and entry_points in egg-info directory
• writes streamlink/plugins/.removed (via options.package_data)
• writes optional data files (shell completions and man pages)
• bakes version info via versioneer

python setup.py develop / pip install -e .

• works

./script/makeinstaller.sh

• builds but didn't test it yet
• writes streamlink/plugins/.removed (via options.package_data)

Please don't merge unless carefully reviewed, as I might have missed something...