Pin pip!=21.3 to avoid pypa/pip#10573 with editable installs

[zklaus]

Description

A recent update to pip introduced a regression that prevents running pip install -e for many projects, see pypa/pip#10573.
Here, we pin pip to avoid the problematic version; a fix is in the works in pypa/pip#10577.

Closes #1358

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[codecov]

Codecov Report

Merging #1359 (a714e80) into main (3c3f5cb) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1359   +/-   ##
=======================================
  Coverage   88.26%   88.26%           
=======================================
  Files         194      194           
  Lines        9836     9836           
=======================================
  Hits         8682     8682           
  Misses       1154     1154           

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[bouweandela]

Could you please do the same for the tool?

[zklaus]

The PR is ESMValGroup/ESMValTool#2344. Just needs a bit of touch up, but working on the core release now.

[valeriupredoi]

cheers @zklaus - sorry, was too late to approve but good thing Bouwe was on the ball 👍