Fix the problem of capslock on macOS

[yrjyrj123]

Hi,

I am a macOS user.
I am using this module to do some interesting things, I think this module is very useful.
But, I found a problem.

If you check the option in the red box in macOS keyboard settings, the following error will appear when the capslock is pressed.

Exception in thread Thread-1:
Traceback (most recent call last):
File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/threading.py", line 917, in _bootstrap_inner
self.run()
File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/threading.py", line 865, in run
self._target(*self._args, **self._kwargs)
File "/Users/hero/venv/dazahui/lib/python3.7/site-packages/keyboard/init.py", line 294, in listen
_os_keyboard.listen(self.direct_callback)
File "/Users/hero/venv/dazahui/lib/python3.7/site-packages/keyboard/_darwinkeyboard.py", line 441, in listen
KeyEventListener(callback).run()
File "/Users/hero/venv/dazahui/lib/python3.7/site-packages/keyboard/_darwinkeyboard.py", line 375, in run
Quartz.CFRunLoopRunInMode(Quartz.kCFRunLoopDefaultMode, 5, False)
File "/Users/hero/venv/dazahui/lib/python3.7/site-packages/keyboard/_darwinkeyboard.py", line 394, in handler
if key_name.endswith("shift") and (flags & Quartz.kCGEventFlagMaskShift):
AttributeError: 'NoneType' object has no attribute 'endswith'

After debugging, I found the reason.
This line of code returned an invalid value 0xFF, I don't know why this happened, but it did happen.
scan_code = Quartz.CGEventGetIntegerValueField(event, Quartz.kCGKeyboardEventKeycode)
https://github.com/boppreh/keyboard/blob/master/keyboard/_darwinkeyboard.py#L375
This API is provided by Apple, but I can't find document about the return value 0xFF, maybe it stands for invalid.

I tried to deal with this invalid value, and now it works well on my macbook.

By the way, I am using macOS 10.15.5.

If you think this is valuable, please merge it.

Thx.

[HaujetZhao]

One of my friends met this problem too!!!