A handful of flake8 fixes
Fixed #21200 -- Consistantly raise errors across all cache backends.

Thanks to tchaumeny for the patch.
Fixed #21486 -- Prevented settings config in signal connection

This was particularly problematic in the chain get_wsgi_application
-> db.connections import -> signal connection -> settings configuration.
Thanks Jon Dufresne for the report.
Follow up to e112654fc81ddb3fbffbb8382b004d69367a85fe

Actually comitted the code now :þ
Don't fail if cPickle doesn't exist.

Today is not my day :(
Fixed test breakage under python 3 introduced by a480f8320a5b36501bfd0e8cd70b8dc04adf2d08.
Add missing commas in Prefetch docs
Merge pull request #1980 from krallin/patch-1

Add missing commas in Prefetch docs
Randomized KEY_PREFIX in caches test to prevent failures during parallel testruns.
Fixed #21505 -- Added unit test for django.utils.text.get_valid_filename.
Fixed regression from ffc37e2343a93cf6d44247e20cd263b41f931716.

This (hopefully) ensures that the cache are created the same way as before
the offending commit.
Don't fail if there is no memcached backend active.