fix(future.py): add tests and fixes

[pcriadoperez]

• Change state lookups to use api instead of _state
• add tests
• use reject and resolve function to avoid InvalidStateErrors
• handle parent cancelation

Existing issues:

• when closing an exchange if a race future has been resolved and not yet created again with the underlying futures, when closing the exchange you will get a message for Future exception was never retrieved
• When cancelling a race future we do not propogate the cancel to it's underlying futures as they may be shared with other race futures, this can cause having underling futures without a parent race and end up never retrieved.

The only way to solve this I can think of is we would have refactor to track what futures are part of race_futures and which ones are not, to know which ones to reject and which ones to cancel. And then keep track of the race_futures seperately to throw an ExchangeClosedByUser in the case they all it's promises are cancelled. But this seems a large change that I'm not sure if we want to go into.

Test by running npm run test-python-future
Test by running python3 python/ccxt/pro/test/base/test_close.py