win32 backend not reenterant

When I'm [using](https://github.com/dschep/ntfy/issues/138) `ntfy` as a library `win32` backend produces following error:
```
ERROR:ntfy:Failed to send notification using default
Traceback (most recent call last):
File "C:\tools\python36\lib\site-packages\ntfy\backends\default.py", line 20, in notify
module.notify(title=title, message=message, **kwargs)
File "C:\tools\python36\lib\site-packages\ntfy\backends\win32.py", line 54, in notify
WindowsBalloonTip(message, title)
File "C:\tools\python36\lib\site-packages\ntfy\backends\win32.py", line 26, in __init__
classAtom = win32gui.RegisterClass(wc)
pywintypes.error: (1410, 'RegisterClass', 'Class already exists.')
```