Give everything some love.

[goodtune]

• updated the CacheLoader and FilesystemLoader to minimally extend the Django versions.
• got the test suite running for Python 2.7, 3.6, 3.7 and Django 1.11, 2.0, 2.1, 2.2, 3.0
• updated the travis-ci configuration to test all of the dimensions
• configured setuptools-scm for packaging from git tags
• removed legacy TEMPLATE_* references (that was dropped in Django 1.8)
• restyled every file I've touched with black (the uncompromising code formatter)
• removed a bunch of old version compatibility shims
• simplified tenant_command

[rj76]

awesome :)