Check that first `typeclass` argument is positional

[sobolevn]

We need this because of how dispatch is called:

classes/classes/_typeclass.py

Line 364 in 7e8ffd1

instance: Union[ # type: ignore

Related: python/mypy@c3ca0d6#diff-ffa623eda70646e703ce29c2bfe71e5d535dd0a0f0b785c094ed5c97cab6cf37R200