py/stream: Use uPy errno instead of system's for non-blocking check.

This is a more consistent use of errno codes.  For example, it may be that
a stream returns MP_EAGAIN but the mp_is_nonblocking_error() macro doesn't
catch this value because it checks for EAGAIN instead (which may have a
different value than MP_EAGAIN when MICROPY_USE_INTERNAL_ERRNO is enabled).