setup: Update the minimum required runtime python3 version

Related to T2367

setup.py

 #!/usr/bin/env python3
-# Copyright (C) 2015-2018  The Software Heritage developers
+# Copyright (C) 2015-2020  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
@@ -42,6 +42,7 @@ setup(
     description="Software Heritage data model",
     long_description=long_description,
     long_description_content_type="text/markdown",
+    python_requires=">=3.7",
     author="Software Heritage developers",
     author_email="swh-devel@inria.fr",
     url="https://forge.softwareheritage.org/diffusion/DMOD/",