Prior to this change, ManyToManyField was declared as being generic in
_ST and _GT, but also used the _T Typevar in its __init__ signature.
This caused mypy to add _T to the variables it was generic in when used
as an alias.
The symptom of this problem was that mypy would show an error with the
message "Type application has too few types (3 expected)" where a
ManyToManyField alias was declared, but adding an extra argument would
fail because the type only takes two arguments.
This change brings the signature of ManyToManyField in line with
ForeignKey and OneToOneField.
* change get_user to use a protocol requiring a session
define a "_HasSession" protocol, and update contrib.auth.get_user to use it
get_user only requires a session field, and idiomatic django testing frequently calls get_user with a TestClient
* run black
* use union for get_user instead of a protocol
* create tests for get_user typechecking
* properly import test client
