Update pytz to 2018.6

Update the pytz library files to those from the 2018.6 release.

lib/pytz/lazy.py

@@ -1,8 +1,11 @@
 from threading import RLock
 try:
-    from UserDict import DictMixin
-except ImportError:
-    from collections import Mapping as DictMixin
+    from collections.abc import Mapping as DictMixin
+except ImportError:  # Python < 3.3
+    try:
+        from UserDict import DictMixin  # Python 2
+    except ImportError:  # Python 3.0-3.3
+        from collections import Mapping as DictMixin
 
 
 # With lazy loading, we might end up with multiple threads triggering
@@ -13,6 +16,7 @@ _fill_lock = RLock()
 class LazyDict(DictMixin):
     """Dictionary populated on first use."""
     data = None
+
     def __getitem__(self, key):
         if self.data is None:
             _fill_lock.acquire()