Better error handling if pytz is missing

See https://github.com/closeio/ciso8601/issues/18

We should probably do a better job at failing in https://github.com/closeio/ciso8601/blob/master/module.c#L245

<del>I think right now it leaves pytz_fixed_offset and pytz_utc uninitialized, which is bad.</del> (it’s a global, so it’s NULL by default)

[batkuip]

Currently, it’s parsing but ignoring timezone information if pytz is not available. So you get a valid date back, but it’s just wrong. This was a nasty bug to track down. On one system it was working fine, the other it wasn’t with the same dates. Thought I was going mental.

[movermeyer]

@dahlia If ciso8601 was written at the Python level (and not the C-API level), then we could make use of datetime.timezone and drop the dependency on pytz for Python 3.2+.

However, datetime.timezone objects are not exposed to the C-API until Python 3.7 😢 . So ciso8601 still requires pytz for all current versions of Python.

I will make the change to have it raise an ImportError if someone tries to parse an aware timestamp with parse_datetime()