diskcache unconditionally imports django if installed #347

Hi, this is bit of a niche issue and I apologize in advance 😅. I do think there's some general merit to improving this though.

Background: We use diskcache on an embedded system based on a Raspberrypi compute module 4. We also have a django app there, but that is a separate process and we don't actually use the django diskcache based cache backend.

I just discovered that when django is installed diskcache unconditionally imports some django machinery for setting up it's DjangoCache. This causes an additonal 500ms startup time for a simple process that uses diskache compared to when django is not present on the system.

Doing the same test on my development machine there difference is < 30ms. But any python process that uses diskcache also uses about 10MB more memory just by django being present.

$ /usr/bin/time -v python -c "import diskcache"
	Command being timed: "python -c import diskcache"
[...]
	Maximum resident set size (kbytes): 24640
[...]
$ pip uninstall django
Found existing installation: Django 5.2.1
Uninstalling Django-5.2.1:
[...]
  Successfully uninstalled Django-5.2.1
$ /usr/bin/time -v python -c "import diskcache"
	Command being timed: "python -c import diskcache"
[...]
	Maximum resident set size (kbytes): 14552
[...]
	Exit status: 0

I'm not sure what the best solution here is but maybe the DjangoCache can be lazily imported when the class is actually imported from another module?