add GitHub URL for PyPi

[andriyor]

Warehouse now uses the project_urls provided to display links in the sidebar on this screen, as well as including them in API responses to help the automation tool find the source code for Requests.

[akx]

Thanks! Looks good to me :)

[codecov]

Codecov Report

Merging #846 (21ba427) into master (9033f02) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #846      +/-   ##
==========================================
- Coverage   91.16%   91.15%   -0.01%     
==========================================
  Files          23       23              
  Lines        4152     4149       -3     
==========================================
- Hits         3785     3782       -3     
  Misses        367      367              

Impacted Files	Coverage Δ
babel/messages/plurals.py	100.00% <ø> (ø)
babel/messages/catalog.py	95.29% <100.00%> (-0.04%)	⬇️
babel/support.py	83.66% <100.00%> (ø)

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