Django 3.0 compatible

[ckleemann]

With this PR secure_mail works with Django 3.0 and newer. Also the support for Python 2 is dropped since Django dropped the support for Python 2 with Version 2.0 in December 2017.

[blag]

Unfortunately I think it's still premature to drop support for Python 2. I can't tell exactly when Django 1.11 reaches end-of-life. The only documentation I have found so far says that Django 1.11 is supported "until at least April 2020", and I haven't found any further update on that.

But we can certainly drop tests for Django < 1.11 and 2.0 - 👍

[ckleemann]

The table of the Django download page lines out that Django 1.11 reached the end of the extended support on April 1, 2020.
So for me it is reasonable to release a version 1.2.0 which drops the support for Python 2.

[blag]

Awesome, thank you for digging into that. Please also update the Travis test matrix to contain only currently supported configurations of Django and Python. That should theoretically be the only necessary fix for CI.

[ckleemann]

I updated the CI matrix but the tests are still failing. I would say the path which uses python-gnupg is not the same as the path of the gpg command. Do you have any ideas about that?

[ckleemann]

I fixed the CI run. The missing path has another name then expected. So the chance is now ready for merge.

[ckleemann]

@blag is there something I can do about this merge request?

[blag]

Sorry! Merged. Thank you for your contribution! 😄

[blag]

I'll fix #9 and push a release to PyPI today.

[blag]

Update: I have fixed #7 and #9 in a local branch, but I ran into issues getting the tests to run locally with #7. I have resolved those issues as well, but I would like to ensure that my changes are tested with both GnuPG v1 and GnuPG v2. I haven't had time to figure out how to test that in Travis yet, but I hope to later this week. Once that's done I'll push the code to GitHub and cut a new release to PyPI.