Switch unittests to pytest with pytest fixtures

[jelly]

Change the unit test system to pytest with it's easy fixture system to replace the current json fixtures.

• Consider using pytest-django
• Add fixtures for mirrorurls etc.
• Django testing documentation

[angvp]

Remember you have this PR https://github.com/archlinux/archweb/pull/42 around :)

[jelly]

wip branch https://github.com/jelly/archweb/commit/bda9a1d197ba70fa9d638a3ca195ece41dfe1516