Remove explicit pip requirement (oldest tests now force 20.3)

[markotoplak]

Issue

Oldest test fails. Now they still fail (timeout when the tests are being executed), but at least they get past this:

pip._vendor.pytoml.core.TomlError: /tmp/pip-install-x83ndwu_/libcst/pyproject.toml(51, 1): array-type-mismatch

Description of changes

require pip>=20.3 (for manylinux_x_y packages like libcst)

See https://github.com/pypa/manylinux

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.76%. Comparing base (9ec84ff) to head (45709bc).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7119      +/-   ##
==========================================
- Coverage   88.76%   88.76%   -0.01%     
==========================================
  Files         334      334              
  Lines       73665    73665              
==========================================
- Hits        65392    65390       -2     
- Misses       8273     8275       +2     

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