Add cancellation tokens support

[pomponchik]

Hello!

I'm looking to the code of this project now, it's very interesting. I suggest you to add support for using of cancellation tokens as well as you can to describe simple timeouts as numbers.

Why could it be useful? If you're using cancellation tokens, you're abstracting the certain reason why the operation can be cancelled. It can be a simple timeout - why not, but also it can be an another reason.

I wrote a library for this kind of stuff - cantok. If it'll be useful, i can advise you how to use it correctly.

filelock/src/filelock/_api.py

Line 264 in 9048580

if 0 <= timeout < time.perf_counter() - start_time: