Drop support for Python 3.3 (#2258)

Closes #2257.

tests/mypy_test.py

@@ -88,7 +88,7 @@ def main():
         print("Cannot import mypy. Did you install it?")
         sys.exit(1)
 
-    versions = [(3, 7), (3, 6), (3, 5), (3, 4), (3, 3), (2, 7)]
+    versions = [(3, 7), (3, 6), (3, 5), (3, 4), (2, 7)]
     if args.python_version:
         versions = [v for v in versions
                     if any(('%d.%d' % v).startswith(av) for av in args.python_version)]