deps: fix build for Python 3.6

Even though Python 3.6 had reached its EOL in 2021-12-23 [1], it doesn't
seem too hard to restore its support for now.

1. https://devguide.python.org/versions/

Author: DifferentialOrange

setup.py

@@ -112,6 +112,7 @@ def get_dependencies(filename):
     command_options=command_options,
     install_requires=get_dependencies('requirements.txt'),
     setup_requires=[
+        'packaging==21.3',
         'setuptools_scm==6.4.2',
     ],
     python_requires='>=3.6',