Remove '<4' from python_requires

[matthewfeickert]

Summary

As discussed in the discuss.python topic "Use of “less-than next-major-version” (e.g., <4) in python_requires (setup.py)" and other places there can be some side effects of adding <4 to a library's python_requires. Following these discussions the use of <4 for python_requires was removed from the PyPA's packaging guide in pypa/packaging.python.org#850. While

requests/setup.py

Line 97 in 61c324d

python_requires=">=3.7, <4",

doesn't pose an immediate breaking threat in 2023, it would be good to remove this upper cap and is using python_requires in a way that was not intended.