Bump pip minimum version to >= 20.3

[atugushev]

Due to pip-tools' policy to support only the last two minor pip versions, it's time to drop the pip-20.1 and pip-20.2.

Latest: 21.0.*
Previous: 20.3.*

Changelog-friendly one-liner: Bump pip minimum version to >= 20.3.

Contributor checklist

• Provided the tests for the changes.
• Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md on release).
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[codecov]

Codecov Report

Merging #1340 (e3b0cf5) into master (4f89da8) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1340      +/-   ##
==========================================
- Coverage   99.62%   99.62%   -0.01%     
==========================================
  Files          33       33              
  Lines        2928     2915      -13     
  Branches      319      313       -6     
==========================================
- Hits         2917     2904      -13     
  Misses          5        5              
  Partials        6        6              

Impacted Files	Coverage Δ
tests/test_resolver.py	100.00% <ø> (ø)
piptools/repositories/pypi.py	97.11% <100.00%> (-0.14%)	⬇️

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[jdufresne]

Only comment is very minor and cosmetic. Otherwise, looks great. Thanks for cleaning this up!

[atugushev]

Thanks @jdufresne for reviewing this! Much appreciated 👍🏼