fix: Remove '<4' from python_requires (#6333)

* As discussed in https://discuss.python.org/t/use-of-less-than-next-major-version-e-g-4-in-python-requires-setup-py/1066
  and other places by the PyPA, use of upper bounds with python_requires
  for future versions of Python is unintended use of python_requires and
  actively discouraged.

Author: matthewfeickert

setup.py

@@ -94,7 +94,7 @@ def run_tests(self):
     package_data={"": ["LICENSE", "NOTICE"]},
     package_dir={"requests": "requests"},
     include_package_data=True,
-    python_requires=">=3.7, <4",
+    python_requires=">=3.7",
     install_requires=requires,
     license=about["__license__"],
     zip_safe=False,