using functools.partial as a class leads to spurious errors

[rchen152]

python/typeshed@e45f443 changed the definition of functools.partial from a class to a function. Things like isinstance() now cause spurious errors.

[tseaver]

@rchen152 Per our discussion on googleapis/google-cloud-python#8106, I think it was the other way around (from a class / type to a function).

[tseaver]

Oops, the relevant discussion is on googleapis/google-cloud-python#8097.

[rchen152]

You're absolutely right - fixed.