gdb/python: remove PyObject_IsTrue call in gdbpy_handle_missing_debuginfo

gdb/python: remove PyObject_IsTrue call in gdbpy_handle_missing_debuginfo

In this review:

  https://inbox.sourceware.org/gdb-patches/87wmirfzih.fsf@tromey.com

Tom pointed out that using PyObject_IsTrue as I was doing, though
technically fine, at least appears to be missing an error check, and
that it would be better to compare to Py_True directly.  I made that
change in the patch Tom was commenting on, but I'd actually copied
that code from elsewhere in python/python.c, so this commit updates
the original code inline with Tom's review feedback.

There should be no functional change after this commit.

Approved-By: Tom Tromey <tom@tromey.com>