Support Django 3.1

[adamchainz]

• Add 3.1 to the tox and travis matrices
• Run tests with deprecation and pending deprecation warnings configured to error. This catches such warnings early rather than passing them on to users.
• Fix the one Django 3.1 pending deprecation warning:

/.../leaflet/templatetags/leaflet_tags.py:93: RemovedInDjango40Warning: force_text() is deprecated in favor of force_str().
  layers=[(force_text(label), url, attrs) for (label, url, attrs) in instance_app_settings.get('TILES')],

[Gagaro]

Thanks :+1: