Handle datetimes with time zone information in crypto.X509Store.set_time()

[orosam]

#907 fixed the issue with set_time() not working on Windows. It also changed set_time()'s behavior in an incompatible way: instead of treating vfy_time always being in local time (regardless if it had a time zone attached or not), it now treats vfy_time as a time in UTC.

This patch improves on that by taking the time zone into account, and also documents the incompatible change.

Note that it is not always possible to convert a timestamp in an arbitrary time zone into UTC unambiguously, due to repeated or skipped local times around DST changes. The best is to use a timezone-aware vfy_time using the UTC time zone.

[orosam]

Two checks appear to be failing due to cryptography no longer allowing OpenSSL 1.0.2.

[alex]

flips table

Ok, I guess we'll need to figure that out before we can review your PR. THank you for sending it!

[alex]

Is it possible to write a test case for this?