In 9.0.x, serial_bytes_available returned a bool and tud_cdc_available was
harmlessly checked twice for any characters.
When the routine was changed to return an int, the double checking led to
over-reporting the number of characters available. In code that would attempt
to read this many bytes from sys.stdin, this made the read call block since
only 1 byte was actually available.
This behavior came up in the discussion of #9393. I don't mark this bug as
closing that one, because that issue seems to be reporting multiple things that
this change would not address, such as delays in `sys.stdout.write()` or
problems seen while using webserial.
Jeff Epler
dc77f3146f