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

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).