Fix incorrect import in Python 3.7+

It turns out that the import of `contextlib.nullcontext` in Python 3.7+
was accidentally trying to import `contextmanager.nullcontext`, but this
was never noticed because it simply fell back to the backport included
here.

I do not know of a great way to test this that doesn't rely on
implementation details, but I have manually tested it by setting a
breakpoint after the import in the `try` statement and importing
`dateutil.tz` manually on Python 3.7, so this version should work.

dateutil/tz/tz.py

@@ -1823,7 +1823,7 @@ def _get_supported_offset(second_offset):
 
 try:
     # Python 3.7 feature
-    from contextmanager import nullcontext as _nullcontext
+    from contextlib import nullcontext as _nullcontext
 except ImportError:
     class _nullcontext(object):
         """