Tighten metaclass __call__ handling in protocols

[ilevkivskyi]

Fixes #19184

This fixes an (edge-case) regression introduced in 1.16. Fix is straightforward, only ignore __call__ if it comes from an actual metaclass.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

nox (https://github.com/wntrblm/nox): 1.27x slower (25.1s -> 32.0s in single noisy sample)

typeshed-stats (https://github.com/AlexWaygood/typeshed-stats): 1.64x slower (57.9s -> 95.1s in single noisy sample)

materialize (https://github.com/MaterializeInc/materialize): 1.05x slower (134.2s -> 141.0s in single noisy sample)

scikit-build-core (https://github.com/scikit-build/scikit-build-core): 1.11x slower (42.7s -> 47.6s in single noisy sample)

discord.py (https://github.com/Rapptz/discord.py): 1.05x faster (306.2s -> 290.3s in single noisy sample)