mavutil: WebSockets: fix wsproto import for newer python versions

[IamPete1]

For reasons I don't really understand the import inside the class init works on Python 3.8.3 where I originally tested but not on Python 3.12.3. This moves the import out of the init which works for both versions. I now have the import in each function of the class that needs it, seems odd, but it does work.

[peterbarker]

pbarker@fx:~/rc/pymavlink((HEAD detached at IamPete1/websockets2))$ mavproxy.py --master /dev/serial/by-id/usb-Hex_ProfiCNC_CubeOrange_3C0028001151383439343731-if00 
Traceback (most recent call last):
  File "/home/pbarker/.local/bin/mavproxy.py", line 4, in <module>
    __import__('pkg_resources').run_script('MAVProxy==1.8.70', 'mavproxy.py')
  File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 656, in run_script
    self.require(requires)[0].run_script(script_name, ns)
  File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 1460, in run_script
    exec(script_code, namespace, namespace)
  File "/home/pbarker/.local/lib/python3.10/site-packages/MAVProxy-1.8.70-py3.10.egg/EGG-INFO/scripts/mavproxy.py", line 39, in <module>
  File "/home/pbarker/.local/lib/python3.10/site-packages/MAVProxy-1.8.70-py3.10.egg/MAVProxy/modules/lib/rline.py", line 8, in <module>
  File "/home/pbarker/.local/lib/python3.10/site-packages/pymavlink-2.4.41-py3.10.egg/pymavlink/mavutil.py", line 1738, in <module>
    class mavwebsocket(mavfile):
  File "/home/pbarker/.local/lib/python3.10/site-packages/pymavlink-2.4.41-py3.10.egg/pymavlink/mavutil.py", line 1740, in mavwebsocket
    from wsproto import ConnectionType, WSConnection, utilities
ModuleNotFoundError: No module named 'wsproto'

... we shouldn't fail that way if someone doesn't have wsproto installed; we try to avoid adding hard dependencies in pymavlink whereever possible.

pbarker@fx:~$ python --version
Python 3.10.12
pbarker@fx:~$ 

[IamPete1]

I have re-worked and re-tested.

[peterbarker]

LGTM