Update test dependencies (tox v4)

[DenverCoder1]

Since tox-gh-actions has a stable release supporting tox v4, both dependencies can be updated.

To test with tox v4, whitelist_externals needs to be replaced with allowlist_externals.

Also updates codecov/codecov-action from v2 to v3.

• https://tox.wiki/en/latest/upgrading.html
• https://github.com/codecov/codecov-action#migration-from-v1-to-v3

[codecov[bot]]

Codecov Report

Merging #953 (c714ad0) into master (304ef12) will increase coverage by 0.30%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #953      +/-   ##
==========================================
+ Coverage   90.18%   90.48%   +0.30%     
==========================================
  Files          24       24              
  Lines        4391     4393       +2     
==========================================
+ Hits         3960     3975      +15     
+ Misses        431      418      -13     

Impacted Files	Coverage Δ
babel/messages/catalog.py	95.52% <0.00%> (+0.02%)	:arrow_up:
babel/localedata.py	95.96% <0.00%> (+0.80%)	:arrow_up:
babel/dates.py	84.97% <0.00%> (+0.94%)	:arrow_up:
babel/localtime/_helpers.py	61.53% <0.00%> (+19.23%)	:arrow_up:

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