ci: toggle colorful test output in pytest.ini

[wpbonelli]

• followup to chore: format tests, colorize pytest CI output #1961
• configure once in pytest.ini instead of --color=yes on every invocation

[codecov]

Codecov Report

Merging #1967 (ddc0b0c) into develop (941a5f1) will decrease coverage by 0.1%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           develop   #1967     +/-   ##
=========================================
- Coverage     72.7%   72.7%   -0.1%     
=========================================
  Files          257     257             
  Lines        57800   56421   -1379     
=========================================
- Hits         42023   41019   -1004     
+ Misses       15777   15402    -375     

see 72 files with indirect coverage changes