Use virtualenv>=20

[kaisen]

virtualenv>=20 contains a big refactor of the inner workings of virtualenv which broke venv-update. This PR accounts for the major changes and requires virtualenv>=20 moving forward.

Notable changes:

• virtualenv>=20 is required
• The old adjust_options hook doesn't exist anymore in virtualenv, so the logic to check if an old virtualenv is valid needs to be done before virtualenv is invoked
• Checking if --system-site-packages was passed to virtualenv is now done by checking for a config in the new pyenv.cfg file
• Major version bump to 4.0.0

Fixes #231.

[asottile]

the invalid check can be greatly simplified as virtualenv 20.x doesn't do major copies of things

here's some prior art: https://github.com/pre-commit/pre-commit/blob/24d9dc7469f9b752b5c772d5cc829d95b43e0e8c/pre_commit/languages/python.py#L166-L186

[ymilki]

Thanks for all the attention to get all the tests passing!