[flake8-pyi] Avoid flagging custom-typevar-for-self on metaclass methods (PYI019)

[vladNed]

Summary

Function custom_type_var_instead_of_self handling PYI019 was modifying return type to Self for metaclasses as the is_metaclass check was missing form implementation.

As discussed in #16127, this is a behavior not accepted by PEP 673

Test Plan

• Tested with a metaclass in order to check that the type is not an issue
• Tested with a normal class to check that Self is indeed outlined when checking with ruff
• Added a metaclass to resources py files and made a new snapshot

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

ℹ️ ecosystem check detected linter changes. (+0 -1 violations, +0 -0 fixes in 1 projects; 54 projects unchanged)

rotki/rotki (+0 -1 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --ignore RUF9 --no-fix --output-format concise --preview

- rotkehlchen/utils/mixins/enums.py:12:16: PYI019 [*] Use `Self` instead of custom TypeVar `T`

Changes by rule (1 rules affected)

code	total	+ violation	- violation	+ fix	- fix
PYI019	1	0	1	0	0

[AlexWaygood]

Thank you! I pushed a very minor simplification, but this is great