Resolves #1753

This adds the man page, if it exists, to wheels via the data_files keyword:
https://setuptools.readthedocs.io/en/latest/references/keywords.html

The data_files keyword is marked as deprecated, but there doesn't seem to be another way of mapping files from one path to another in the distribution file. What it says about not being supported by wheels is wrong. package_data doesn't seem to work here without changing build scripts. At least I wasn't able to make it work.

sdist does not include any data_files.

Regarding the manpages themselves, it might be necessary on some systems to extend the MANPATH env var with MANPATH="${XDG_DATA_HOME:-$HOME/.local/share}/man:$MANPATH" so that it looks up man pages in $XDG_DATA_HOME/man (which defaults to $HOME/.local/share/man) too.

To validate the changes:

$ make --directory=docs clean
$ python setuptools.py bdist_wheel
$ unzip -l dist/streamlink-2.0.0+10.gc6a1806-py3-none-any.whl | grep share/man/man1/streamlink.1

$ make --directory=docs clean man
$ python setuptools.py bdist_wheel
$ unzip -l dist/streamlink-2.0.0+10.gc6a1806-py3-none-any.whl | grep share/man/man1/streamlink.1
    49409  2020-12-28 12:05   streamlink-2.0.0+10.gc6a1806.data/data/share/man/man1/streamlink.1
$ pip install ./dist/streamlink-2.0.0+10.gc6a1806-py3-none-any.whl
$ ls ~/.local/share/man/man1/streamlink.1 
/home/basti/.local/share/man/man1/streamlink.1