[3.15] gh-149044: Improve Py_tp_base[s] docs & error message for non-type bases ...

[3.15] gh-149044: Improve Py_tp_base[s] docs & error message for non-type bases (GH-151252) (GH-151606)

gh-149044: Improve Py_tp_base[s] docs & error message for non-type bases (GH-151252)

The initial implementation of PEP 820 worsened the error message
when non-types are given as base types in Py_tp_bases & Py_tp_base.
Bring back the 'bases must be types' wording and add a 'got' note for
easier debugging.

Improve slot ID documentation, and soft-deprecate Py_tp_base
(as per the PEP).
(cherry picked from commit 16185e9fe2037d2171626f79c3d099bd7772b53e)

Co-authored-by: Petr Viktorin <encukou@gmail.com>