ci: colorize tox output

[mr-cal]

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• Have you successfully run tox?

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Colorize the output of tox in Github CI.

Tox does not colorize its output in Github because of the envvar TERM is set to dumb. However, github handles colorized output quite nicely.

Before
After

[codecov]

Codecov Report

Merging #442 (8e62f2f) into main (df23ecc) will decrease coverage by 0.01%.
The diff coverage is n/a.

❗ Current head 8e62f2f differs from pull request most recent head 82ef6fb. Consider uploading reports for the commit 82ef6fb to get more accurate results

@@            Coverage Diff             @@
##             main     #442      +/-   ##
==========================================
- Coverage   99.00%   99.00%   -0.01%     
==========================================
  Files          57       57              
  Lines        4532     4527       -5     
==========================================
- Hits         4487     4482       -5     
  Misses         45       45              

see 1 file with indirect coverage changes

[lengau]

Oooh pls upstream this to starbase!

[mr-cal]

Oooh pls upstream this to starbase!

Already done! canonical/starbase#145