PosixPollSerial.read fails when timeout=None (the default)

[mvgrad98]

If you do not use a timeout (the default) then the conversion to ms from seconds fails:
TypeError: unsupported operand type(s) for *: 'NoneType' and 'int'

pyserial/serial/serialposix.py

Line 742 in 797018b

for fd, event in poll.poll(self._timeout * 1000):

Oddly there is already support for _timeout being None following the poll() call but I can't see how we would ever make it there if _timeout was None.

[zsquareplusc]

i guess that *1000 came in later as the poll function does not take seconds as otherwise usual in Python APIs..

now using the Timeout helper class and i've added support for cancel_read() while at it