Improve package metadata

[cthoyt]

This pull request does three things:

1. Updates an old key in setup.cfg so the README.md file is properly displayed on PyPI (i.e., switch from description-file to the pair of long_description/long_description_content_type)
2. Adds application of pyroma to the linting configuration for GitHub Actions to check the package metadata is completely filled out
3. Fill out the missing package metadata in setup.cfg

A little context, if you run pyroma on the current PyPI version of PyG:

$ pyroma -p torch_geometric
------------------------------
Checking torch_geometric
Starting new HTTPS connection (1): pypi.org:443
https://pypi.org:443 "GET /pypi/torch_geometric/json HTTP/1.1" 301 223
https://pypi.org:443 "GET /pypi/torch-geometric/json HTTP/1.1" 200 5146
Found torch_geometric version 2.0.2
Downloading torch_geometric-2.0.2.tar.gz to verify distribution
Starting new HTTPS connection (1): files.pythonhosted.org:443
https://files.pythonhosted.org:443 "GET /packages/05/52/b0bf572b72fb3fd0b57eabd3c46f25d128579c586dfbe25cc4f9d4163306/torch_geometric-2.0.2.tar.gz HTTP/1.1" 200 325559
/Users/cthoyt/.virtualenvs/pykeen/lib/python3.9/site-packages/setuptools/dist.py:717: UserWarning: Usage of dash-separated 'description-file' will not be supported in future versions. Please use the underscore name 'description_file' instead
  warnings.warn(
Found torch-geometric
------------------------------
The package's long_description is quite short.
Your package does not have classifiers data.
You should specify what Python versions you support.
Your package does neither have a license field nor any license classifiers.
You should have three or more owners of the project on PyPI.
Specifying a development status in the classifiers gives users a hint of how stable your software is.
------------------------------
Final rating: 6/10
Comté
------------------------------

[codecov-commenter]

Codecov Report

Merging #3445 (9e4d58e) into master (6961b16) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3445   +/-   ##
=======================================
  Coverage   81.70%   81.70%           
=======================================
  Files         291      291           
  Lines       14459    14459           
=======================================
  Hits        11814    11814           
  Misses       2645     2645           

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[rusty1s]

This is great! Thank you so much.