add new "exclude count" page number pagination option

[aleontiev]

...to support a faster page-number pagination over large datasets

• Like standard PageNumberPagination, except the results are never counted
• Uses the N+1 trick from CursorPagination to determine whether the dataset has more pages
• Can be enabled on any request via query parameter exclude_count (name can be configured)

TODO:

• [x] ensure all current tests pass
• [x] add new test case with exclude_count enabled

[aleontiev]

@chdastolfo @suavesav

• all tests pass including the new one I added
• addressed all PR feedback
• bumped version to 2.1.3 in setup.py

so I am going to merge 🚀