gdb/testsuite: address "unused not at top of file" warnings in Python files

Address flake8 warnings like this one:

    gdb/testsuite/gdb.python/py-typeprint.py:60:1: E402 module level import not at top of file

Change-Id: I6ab7880ec5b55b6a5c85cb01d0f85172d44b4ee1
Approved-By: Tom Tromey <tom@tromey.com>
4 files changed