Add project URLs into the package metadata for convenience links on Pypi

[adswa]

This PR adds optional package metadata in the form of project_urls. I believe they would show up on pypi.org in the side bar, making it easier to find documentation, source, or bug tracker. Fixes #5865

[codecov]

Codecov Report

Merging #5866 (dd3266b) into maint (7f543d0) will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            maint    #5866      +/-   ##
==========================================
+ Coverage   90.28%   90.34%   +0.06%     
==========================================
  Files         300      300              
  Lines       42370    42390      +20     
==========================================
+ Hits        38252    38297      +45     
+ Misses       4118     4093      -25     

Impacted Files	Coverage Δ
datalad/downloaders/http.py	79.15% <0.00%> (-1.29%)	⬇️
datalad/core/distributed/tests/test_push.py	98.90% <0.00%> (-0.22%)	⬇️
datalad/distribution/tests/test_get.py	100.00% <0.00%> (ø)
datalad/plugin/tests/test_addurls.py	99.54% <0.00%> (+<0.01%)	⬆️
datalad/distribution/get.py	97.92% <0.00%> (+0.01%)	⬆️
datalad/tests/test_utils.py	96.46% <0.00%> (+0.13%)	⬆️
datalad/core/distributed/tests/test_clone.py	97.37% <0.00%> (+0.13%)	⬆️
datalad/support/sshconnector.py	87.81% <0.00%> (+0.35%)	⬆️
datalad/support/tests/test_annexrepo.py	97.48% <0.00%> (+0.43%)	⬆️
datalad/tests/utils.py	89.39% <0.00%> (+0.61%)	⬆️
... and 5 more

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[yarikoptic]

@jwodder -- please share your wisdom on this PR -- will it result in desired effect on pypi?

[jwodder]

@yarikoptic Yes.

[yarikoptic]

uff, all green now, let's proceed!