Remove Py_TPFLAGS_CHECKTYPES

According to 'git annotate', the Py_TPFLAGS_CHECKTYPES was added to
python-internal.h way back when gdb was first ported to Python 3.  It
was a compatibility fix for Python 2.

This is not needed any more, because Python 2 is no longer supported.
This patch removes the vestiges.

Approved-By: Simon Marchi <simon.marchi@efficios.com>