Add python_requires to setup.cfg (#211)

* Add python_requires to setup.cfg

Closes #210

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: sigmavirus24

setup.cfg

@@ -27,6 +27,7 @@ py_modules = flake8_import_order
 install_requires =
     pycodestyle
     setuptools
+python_requires > = 3.9
 tests_require =
     pytest
     flake8