mktime takes local time, not UTC

GPG signature timestamps are considered UTC, this may cause times to be off
by delta > 1000 in def test_signature_string_verification because it
incorrectly gave gmtime() as an argument to mktime().

Python 2 and 3 docs both say that time.mktime() should have a struct_time argument
"which expresses the time in local time, not UTC". So the test is now using
time.localtime() instead of time.gmtime()