devel/py-qt5-sip: Fix build with py311 (+)

The _frame struct was moved to an internal header, however the public API is
primarily read-only, and py-sip needs to build PyFrameObjects so still import
the internal headers.

Also sets the Py_BUILD_CORE define for py311a6, trying to restrict it to the frame header.

Inspired by: cython repo
With hat: kde
Sponsored by: Netzkommune GmbH