Update type annotations and signatures to match djangorestframework-stubs

I noticed this because my virtualenv ended up with the djangorestframework-stubs
package installed, which made the mypy pre-commit hook fail.

This commit adds drf-stubs to the tox.ini, and does the relevant updates to our
type annotations/signatures.

Turns out that upstream DRF has all request parameters named that way, so we
rename all our req parameters to match, which appeases mypy's signature
checks.