test: Expand testing for wheel multi-version multi-platform

[jacobdr]

Following this past week's issue with wheel==0.38, this adds slightly more testing to help guard against a similar issue in this package. While its somewhat duplicative of what might already be tested in test_tags.py, since this is the most natural place to put an "end-user" consumption test (viz the internals), I thought the juice was worth the squeeze as a defensive test.

I submitted a PR with similar testing to the wheel package via https://github.com/pypa/wheel/pull/486, but since the intention is to have wheel depend on packaging eventually, I also wanted to make a similar contribution here.

• Add slightly more testing to the wheel and sdist tests
• Add venv to the .gitignore

[brettcannon]

@jacobdr please refresh my review when you're ready for me to have another look.