Fixes tzname return type

Closes matplotlib#7630

Author: Kojoley

lib/matplotlib/dates.py

@@ -159,7 +159,7 @@ def utcoffset(self, dt):
         return datetime.timedelta(0)
 
     def tzname(self, dt):
-        return "UTC"
+        return str("UTC")
 
     def dst(self, dt):
         return datetime.timedelta(0)

lib/matplotlib/tests/test_dates.py

@@ -465,6 +465,11 @@ def test_DayLocator():
    mdates.DayLocator(interval=1.0)
 
 
+def test_tz_utc():
+    dt = datetime.datetime(1970, 1, 1, tzinfo=mdates.UTC)
+    dt.tzname()
+
+
 if __name__ == '__main__':
     import nose
     nose.runmodule(argv=['-s', '--with-doctest'], exit=False)