toxgen: Retry & fail if we fail to fetch PyPI data

[sentrivana]

• try to refetch data if PyPI returns an error
• if we fail after 3 tries, fail the whole script (it doesn't make sense to run it without access to up-to-date PyPI data)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.51%. Comparing base (adcfa0f) to head (72aabf3).
Report is 2 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4251      +/-   ##
==========================================
- Coverage   79.54%   79.51%   -0.04%     
==========================================
  Files         142      142              
  Lines       15898    15898              
  Branches     2721     2721              
==========================================
- Hits        12646    12641       -5     
- Misses       2388     2390       +2     
- Partials      864      867       +3     

see 5 files with indirect coverage changes