Remove <4.0 Python Requirement

[rbebb]

Since flake8-eradicate uses ^3.8 for its Python version range, this requires downstream projects that use poetry to set <4.0 as an upper bound for Python. It would be great if the Python version range could be changed to >=3.8 so that the upper bound isn't propagated downstream.

[sobolevn]

What is the reason not to do that? We (even I) don't know anything about Python4, it is safe to assume that it will break quite some things.