gdb/testsuite: use "not in" in Python files
Fix flake8 warnings like this one:
gdb/testsuite/gdb.python/py-send-packet.py:68:12: E713 test for membership should be 'not in'
Change-Id: I2b4cc1eeb63ee2fceb8c4264e7d6ce2d22824688
Approved-By: Tom Tromey <tom@tromey.com>