Improve transport types

[alexrudd2]

The first commit is straightforward.

For the second one, mypy complains that we are passing asyncio.CancelledError to close which only takes Exception. As of Python 3.8 asyncio.CancelledError is dervied from BaseException instead. It will therefore not be caught or suppressed by anything acting on Exception.

pymodbus/pymodbus/transport/serialtransport.py

Lines 53 to 55 in 7c4a310

	if exc:
	with contextlib.suppress(Exception):
	self.intern_protocol.connection_lost(exc)

I don't see any use in passing it, so I just used None. The alternative is to just type: ignore

[janiversen]

LGTM, thanks,