Issue #10941: Fix imaplib.Internaldate2tuple to produce correct result near

the DST transition.  Patch by Joe Peterson.

Lib/test/support.py

@@ -53,7 +53,7 @@ __all__ = [
     "reap_children", "cpython_only", "check_impl_detail", "get_attribute",
     "swap_item", "swap_attr", "requires_IEEE_754",
     "TestHandler", "Matcher", "can_symlink", "skip_unless_symlink",
-    "import_fresh_module", "failfast",
+    "import_fresh_module", "failfast", "run_with_tz"
     ]
 
 class Error(Exception):
@@ -1020,6 +1020,35 @@ def run_with_locale(catstr, *locales):
         return inner
     return decorator
 
+#=======================================================================
+# Decorator for running a function in a specific timezone, correctly
+# resetting it afterwards.
+
+def run_with_tz(tz):
+    def decorator(func):
+        def inner(*args, **kwds):
+            if 'TZ' in os.environ:
+                orig_tz = os.environ['TZ']
+            else:
+                orig_tz = None
+            os.environ['TZ'] = tz
+            time.tzset()
+
+            # now run the function, resetting the tz on exceptions
+            try:
+                return func(*args, **kwds)
+            finally:
+                if orig_tz == None:
+                    del os.environ['TZ']
+                else:
+                    os.environ['TZ'] = orig_tz
+                time.tzset()
+
+        inner.__name__ = func.__name__
+        inner.__doc__ = func.__doc__
+        return inner
+    return decorator
+
 #=======================================================================
 # Big-memory-test support. Separate from 'resources' because memory use
 # should be configurable.