Problem

In tzinfo_fromutc(), the result of add_datetime_timedelta() is cast
to PyDateTime_DateTime* without a type check. If a datetime subclass
__new__ returns a non-datetime object, the interpreter crashes with
SIGABRT due to out-of-bounds memory reads inside normalize_y_m_d().

Fix

• Added PyDateTime_Check() after both calls to add_datetime_timedelta()
  in Modules/_datetimemodule.c - raises TypeError instead of crashing
• Added equivalent isinstance() checks in Lib/_pydatetime.py
• Added regression test in Lib/test/datetimetester.py

Testing

Regression test passes for both TestDateTimeTZ_Pure and TestDateTimeTZ_Fast.