Update requirements-windows.txt

- Add pypiwin32 for cx-freeze as it was issuing a warning
- Prevent version 5.1.0 of cx-freeze as it produces non-working
  packages for some reason.  Version 5.0.2 and 6.0b1 work fine.

requirements-windows.txt

@@ -1,2 +1,3 @@
 PyQt5 >=5.4,<6.0
-cx-Freeze>=5.0.2,<6.0.0
+cx-Freeze>=5.0.2,!=5.1.0
+pypiwin32>=200