Drop support for EOL dependencies

[matthewhegarty]

Problem

Drops support for the following which are past EOL. This was reviewed in #1366 for inclusion in 3.0 release. This release pushes it into the main repo for inclusion in 2.8.0.

• python 3.6
• django 2.2, 3.0, 3.1
• Remove skipped test
  • I checked the history of this test and could not confirm that it was serving any useful purpose, so decided to remove.
• Removed import of JSONField from django.contrib.postgres.fields because this is included in Django since 3.1

[coveralls]

Coverage remained the same at 98.39% when pulling 663af80 on matthewhegarty:drop-old-version-support into cdcfa9e on django-import-export:main.