Simplify PyInit_timezone. (GH-9467)

Reduce the knotty preprocessor conditional logic, dedent unnecessarily nested
code, and handle errors properly.

The first edition of this change (afde1c1a05cc8a1e8adf6403c451f6708509a605)
failed (bpo-34715) because FreeBSD doesn't define the timezone globals. That's
why we're now checking for HAVE_DECL_TZNAME.