Removed upper bound on Python version

This is unnecessary (Python is not SemVer'ed) and breaks dependency solvers.

See https://github.com/orgs/python-poetry/discussions/3757#discussioncomment-435337

setup.py

@@ -59,7 +59,7 @@
       ]),
       include_package_data=True,
       zip_safe=False,
-      python_requires='>=3.7, <4',
+      python_requires='>=3.7',
       install_requires=install_requires,
       tests_require=tests_require,
       extras_require={