setup: update python_requires to comply with PEP 345/440

According to PEP 345 Requires-Python
(https://www.python.org/dev/peps/pep-0345/#requires-python), the value
of this field must be a valid Version Specifier
(https://www.python.org/dev/peps/pep-0345/#version-specifiers). Which
in turn expects this to comply with PEP 440
(https://www.python.org/dev/peps/pep-0440/).

While not an issue for those that directly use `pip`, this will cause
issues for `poetry` users in the next release (if their current stance
is maintained). Discussion of the issue and there stance can be found
here: https://github.com/python-poetry/poetry/issues/4095.

setup.cfg

@@ -46,7 +46,7 @@ project_urls =
 [options]
 packages = find:
 
-python_requires = >=3.5.*
+python_requires = >=3.5
 
 include_package_data = True
 install_requires =