fix type hint for GenericSitemap(info_dict)

[djbrown]

Description

This PR softens the invariant type hint Dict to the covariant Mapping of django.contrib.sitemaps.GenericSitemap(info_dict).
See Python type hints: typing.Mapping vs. typing.Dict - Stack Overflow
Docs: The sitemap framework # GenericSitemap

Motivation

info_dict = {
    'queryset': Entry.objects.all()
}

urlpatterns = [
    path('sitemap.xml', sitemap,
         {'sitemaps': {'blog': GenericSitemap(info_dict)}},
         name='django.contrib.sitemaps.views.sitemap'),
]

Mypy error:

error: Argument 1 to "GenericSitemap" has incompatible type "Dict[str, _QuerySet[Entry, Entry]]"; expected "Dict[str, Union[datetime, _QuerySet[Model, Model], str]]"  [arg-type]
note: "Dict" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
note: Consider using "Mapping" instead, which is covariant in the value type

Test

In my local virtualenv I modified the file site-packages/django-stubs/contrib/sitemaps/__init__.pyi according to the change of this PR.
Afterwars ran the mypy check again and the errors was gone.

[adamchainz]

LGTM, this will also allow other kinds of mappings if necessary.

[djbrown]

@adamchainz is this mergable or am I missing something?

[adamchainz]

I don't have merge rights, @sobolevn does.

[sobolevn]

Sorry, I've missed this PR somehow. Let's try it.

@adamchainz if you want to have merge rights drop me an email :)