Resolves #3507

This adds the man page to the sdist tarballs by white-listing the man page build path. It can be found here now:
docs/_build/man/streamlink.1

Due to the white-listing, the man page gets included twice (same path though), first by the recursive-include docs * pattern in the manifest, and second by the data_files field in setup.py (which was added by #3459 for the bdist target).

I would have preferred having the man page in the tarball's root directory or in share/man/man.1/, like the wheels, but that would have required copying the file via script/build-and-sign.sh first and deleting it again afterwards. Unfortunately, the data_files field which moves the man page to share/man/man.1/ in the wheels doesn't work with distutil's sdist command, and it simply ignores the path override, as you can see here:
https://github.com/python/cpython/blob/v3.9.1/Lib/distutils/command/sdist.py#L304-L307

$ python setup.py sdist
$ tar -tzf dist/streamlink-2.0.0+25.g815262d.tar.gz | grep 'streamlink.1'
streamlink-2.0.0+25.g815262d/docs/_build/man/streamlink.1

On the CI side, everything is already in place from the bdist-manpage PR:

• streamlink/.github/workflows/main.yml

  Lines 155 to 156 in c5791f7

  	- name: Build man page
  	run: make --directory=docs man

• streamlink/.github/workflows/main.yml

  Lines 165 to 168 in c5791f7

  	- name: sdist and wheels
  	env:
  	RELEASE_KEY_PASSPHRASE: ${{ secrets.RELEASE_KEY_PASSPHRASE }}
  	run: ./script/build-and-sign.sh