Don't test Django main branch using python36,37

[manelclos]

Problem

Travis is trying to run tests using python36,37 against Django main, which only supports python38,39

Solution

Exclude from matrix

Acceptance Criteria

Tests should pass now

[coveralls]

Coverage remained the same at 96.757% when pulling dd5f0ce on fix-travis-tests into 8e6c0e6 on master.

[andrewgy8]

Thanks @manelclos ! I would really like to move to github actions soon. This travis build takes farrrr too long.

[manelclos]

@andrewgy8 great, I was going to ask that next, but wanted to fix travis to pass cleanly. Just pushed the last update.

[manelclos]

@andrewgy8 tests passed, merging this one. Django main seems to be failing, probably due to postgresql support:

https://github.com/django/django/blob/76c0b32f826469320c59709d31e2f2126dd7c505/django/db/backends/postgresql/introspection.py#L51

https://docs.djangoproject.com/en/dev/ref/databases/#postgresql-notes

We should take care of that when moving to GitHub actions. I don't have much time now. Do you plan to take on it?