Fixed the PostGIS circular imports caused by 1506c71a95.

Thanks to @loic for the help and @timgraham for the review.

refs #12030.
Fixed #22114 -- Stopped adding trailing slashes in URLField.to_python

Thanks coredumperror at gmail.com for the report and Tim Graham
for the review.
Forgotten slash removal in d320863878
Add a useful stacklevel to some RemovedInDjango19Warnings
Skipped a test that errors rather than marked it as an expectedFailure.

The test throws an error which Python 3.4 doesn't seem to catch as an
expectedFailure.

refs 7476d96f83a004d674244aeb7a66289035427396
Added Python 3.4 support notes.