PyType_From*: Disallow metaclasses with custom tp_new

[encukou]

Following up after #60074 and #103968:
In 3.14, the deprecation period is over and PyType_From* should fail if the metaclass has custom tp_new. That means the tp_new is no longer silently skipped.

The proper way to instantiate such a metaclass is to call it.

Linked PRs

• gh-123909: PyType_From*: Disallow metaclasses with custom tp_new #123947
• gh-123909: Remove obsolete note in PyType_FromMetaclass docs #131506