feat(django): Add db.system

[AbhiPrasad]

ref getsentry/team-webplatform-meta#60

So worked out when testing locally + the types seem to enforce this, and it works on all django versions, so I think this is good to go.

[AbhiPrasad]

@antonpirker this is now ready to review - Lint error are unrelated

sentry_sdk/integrations/flask.py:162: error: "ImmutableMultiDict" expects no type arguments, but 2 given  [type-arg]
sentry_sdk/integrations/flask.py:166: error: "ImmutableMultiDict" expects no type arguments, but 2 given  [type-arg]
Found 2 errors in 1 file (checked 91 source files)

https://github.com/getsentry/sentry-python/actions/runs/4809397087/jobs/8560589799?pr=2040

[antonpirker]

lgtm