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 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()
mktime takes localtime, or we skew the clocks

The following snippet shows that to create a proper timestamp, we should
feed mktime() with localtime() data rather than gmtime(), as it is locale
aware.

>>> from time import mktime, gmtime, localtime
>>> bad    = gmtime(mktime(gmtime()))
>>> good   = gmtime(mktime(localtime()))
>>> judge  = gmtime()
>>> if bad == good: "You're using UTC. Demonstration is impossible."
... elif bad == judge: "I can be wrong"
... elif good == judge: "but this shows I'm right"
...
"but this shows I'm right"

The above code checks whether gmtime() == gmtime(timestamp) from
mktime(). Obviously we don't get the same 'now' if we feed the bad value
to mktime.
Merge remote-tracking branch 'mmn/mktime_takes_localtime_not_gmtime' into develop

 * FIX struct_time inputs to time.mktime() to use localtime() not gmtime().
 * CLOSES #9: "mktime takes local time, not UTC"
   https://github.com/isislovecruft/python-gnupg/pull/9