Unlike `List`, which is invariant, `Sequence` is covariant, which lets
`path` accept lists of subsets of the `Union` as well.
I believe this is safe, as django doesn't mutate this input. I found
[this
comment](https://github.com/python/mypy/issues/3351#issuecomment-300447832)
helpful
* Add URLPattern options to django.url.conf since they were missing.
* Fix some wording in the contribution guide & fix a reference to Django version 3.0, which has since been replaced by 3.1 as new stable version.
* My bad - I was misreading the type hints a bit here. Fix tests.
* We need to specify the List[Union[URLResolver, URLPattern]] upfront, since those don't accept List[URLResolver] as argument.
* Add test to ensure that path() accepts a mix of URLPatterns & URLResolvers.
Co-authored-by: LanDinh <coding+sourcetree@khaleesi.ninja>