strptime and time zones

[ghost]

strptime is not handling time zones correctly. For example, if you use the format string "%FT%T%z" with ISO 8601 strings like
1999-05-31T13:20:00-04:00
and
1999-05-31T13:20:00-05:00
you get identical results back.

The code on line 510 seems seems to match the time zone offset but it only does something when the offset if zero.

Imported from rust-lang/rust#2637

[LorenVS]

This seems to be the result of the %z parsing code in strptime only supporting timezones without the colon (i.e. 1999-05-31T13:20:00-0500). RFC-822 does include the colon in its specification. I can't seem. I also see some references (i.e. this stack overflow question http://stackoverflow.com/questions/26165659/python-timezone-z-directive-for-datetime-strptime-not-available) which use the format without the colon.

Should we support both of these or only one?