Remove support for LegacyVersion

[jaraco]

In #2466 and pypa/packaging#368, it's become apparent that Setuptools should get out of the business of supporting the LegacyVersion.

[yan12125]

I had a look into the debt/remove-legacy-version branch - the first failure (Invalid version: '51.1.0.post20201221.post-20201221') is because setuptools itself generates non-PEP440 versions. Other issues are from the package_index module, which accepts non-PEP440 versions. Seems package_index is used by easy_install only? If so, dropping LegacyVersion might be related to dropping easy_install: #917.

With some patches (https://github.com/yan12125/setuptools/commits/debt/remove-legacy-version), the number of failed tests goes down to 3: https://github.com/yan12125/setuptools/runs/1607790124. Notably, test_pbr in test_integration.py failed because there is a non-PEP 440 version of pbr https://pypi.org/project/pbr/0.5.2.5.g5b3e942/ and package_index picks it.

[yan12125]

There is an ongoing PR on removing LegacyVersion: pypa/packaging#407