More fields

[aleksanb]

This pr adds __get__ signatures for even more fields which are used in our codebase.

[aleksanb]

DecimalField should subclass Field and not IntegerField, so i changed it.

[aleksanb]

Unsure how to proceed with the lack of Psychopg stubs, we probably want stubs for the following two files at least: https://github.com/psycopg/psycopg2/blob/master/lib/extras.py and https://github.com/psycopg/psycopg2/blob/master/lib/_range.py, but unsure as to how this would be integrated.

[mkurnikov]

Shouldn't psycopg2 stubs be added in their own psycopg2-stubs package in pypi? And we'll just depend on it.

[aleksanb]

Probably, yes. I didn't really want to be maintaining a separate repo for stubs that I'd only consume through django-stubs, though.

[mkurnikov]

Can it be fixed with # type: ignore on the psycopg2 import line? I don't want to add psycopg2 stubs in this repo, they really do not belong here.

[aleksanb]

@mkurnikov updated with type: ignore for now.

[mkurnikov]

Thanks!