Add project metadata to PyPI.org webpage

Created by: althonos

Hi Mike (or anyone reading this),

since the new PyPI supports Markdown READMEs, I added the appropriate settings to setup.cfg so that the PyPI page actually displays something.

I also added some project links (that are displayed on the left-hand side of https://pypi.org/project/av/), currently there's only Homepage but I also added:

• Bug Reports with a link to the GitHub issue tracker
• Documentation with a link to the docs
• Feedstock with a link to the av conda-forge feedstock

I also added a license key to the metadata, as for now https://shields.io fails to extract the license from the classifiers. This means the license badge for av, which currently is: should look like this once the PR is merged: