[bleak 0.18] Update bleak backend to handle updated callback type in start_notify

[jdpigeon]

This PR addresses the breaking change in bleak 0.18 of the start_notify function's callback returning an object instead of an int.

[xloem]

Thanks so much for doing this. It seems it would be good to merge it.

Possible further improvements:

• The dependency on bleak could specify bleak>=0.18.0 for seamless installs at line 43 https://github.com/alexandrebarachant/muse-lsl/blob/c9f805782f3b91c1a954ad4087211e9a33563e4c/setup.py#L43
  • Before discovering your patch, alternatively to >=0.18.0 I today wrote a fix that detects the version and implements different functions depending on it: https://github.com/alexandrebarachant/muse-lsl/compare/master...xloem:muse-lsl:bleak018?expand=1#diff-637fb59473b44003aa46cc9f9ef991eb649979d35c418e2bfec409bc241deb9e basically the same thing though