Added github-actions for testing on all django versions

[leogregianin]

Added Github Actions for testing on all Python and Django versions.

• Python 2.7: Django 1.6, 1.7, 1.8, 1.9, 1.10, 1.11
• Python 3.6, 3.7: Django 1.8, 1.9, 1.10, 1.11, 2.0, 2.1, 2.2, 3.0, 3.1, 3.2
• Python 3.8, 3.9: Django 2.2, 3.0, 3.1, 3.2, 4.0
• Python 3.10: Django 3.2, 4.0