[WIP] Bump Django to 2.1
A first step to get out of Repomaker's dependency hell. With this MR, we're now back to the newest stable
versions of Django, letting us work with the newest versions of Python again.
Note that we need to do this soon because the Flatpak base is EOL: https://github.com/KDE/flatpak-kde-runtime/commit/4d7a8d72983d9e86f5f5ce46d3524544c0ee6abe
Edit: Newer versions of Django 1.x also support latest Python versions, therefore I've migrated Repomaker to that first: !214 (merged)
Related to fdroid-repomaker-flatpak!29 (closed) and org.fdroid.Repomaker#4.
Fixes #186.