Drop pip-20.0 support

[atugushev]

Due to pip-tools' policy to support only the last two minor pip versions, it's time to drop the pip-20.0.

• pin pip >= 20.1 in setup.py
• remove 20.0 compatible code
• remove 20.0 from CI
• update "Versions and compatibility" in README

Changelog-friendly one-liner: Drop pip-20.0 support.

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).

[AndydeCleyre]

Great, thanks! One question for now, and two for the future.

now:

• Should we remove BAR_TYPES from _compat/{pip_compat,__init__}.py and import it directly in pypi.py?

future:

• Do you think it would be worth the trouble to template some of these files, for more automated pip-version updates? Specifically: .appveyor.yml, ci.yml, .travis.yml, setup.cfg, tox.ini
• Do we have special reasons to stick with setup.{cfg,py} for now, rather than moving to pyproject.toml?

[atugushev]

@AndydeCleyre

• Should we remove BAR_TYPES from _compat/{pip_compat,__init__}.py and import it directly in pypi.py?

Yeah, thanks for noticing!

• Do you think it would be worth the trouble to template some of these files, for more automated pip-version updates? Specifically: .appveyor.yml, ci.yml, .travis.yml, setup.cfg, tox.ini

Would be nice to automate this.

• Do we have special reasons to stick with setup.{cfg,py} for now, rather than moving to pyproject.toml?

I'd prefer to stick to setup.cfg since pip doesn't support install editables with pyproject.toml.

[atugushev]

Thank you @AndydeCleyre for reviewing this! Much appreciated!