Test pip against the main default branch

[atugushev]

Fixes failing cron jobs due to pip renamed the default branch to main. See pypa/pip#8948.

[codecov]

Codecov Report

Merging #1374 (986e00e) into master (2dd787a) will not change coverage.
The diff coverage is n/a.

❗ Current head 986e00e differs from pull request most recent head 388655a. Consider uploading reports for the commit 388655a to get more accurate results

@@           Coverage Diff           @@
##           master    #1374   +/-   ##
=======================================
  Coverage   99.65%   99.65%           
=======================================
  Files          33       33           
  Lines        2940     2940           
  Branches      308      308           
=======================================
  Hits         2930     2930           
  Misses          5        5           
  Partials        5        5           

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[atugushev]

@webknjaz thanks!