Fix incorrect import in Python 3.7+

changelog.d/963.bugfix.rst

@@ -0,0 +1,2 @@
+Fixed issue where ``dateutil.tz`` was using a backport of ``contextlib.nullcontext``
+even in Python 3.7 due to a malformed import statement.

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):
         """