imapclient.fixed_offset.FixedOffset cannot be unpickled

[freddrake]

With a pickled imapclient.fixed_offset.FixedOffset timezone with an offset of datetime.timedelta(0), unpickling raises an exception:

TypeError: __init__() missing 1 required positional argument: 'minutes'

I can work around this by adapting the timestamps returned for INTERNALDATE to use datetime.timezone instances instead of FixedOffset instances, but this shouldn't be necessary.

[mjs]

Bummer. It should be fairly straightforward to allow FixedOffset instances to be unpickled.

Additionally, when running on Python 3, IMAPClient should probably be using datetime.timezone instead of FixedOffset.