Add open() and close() to SerialBase

[jrast]

The SerialBase should have the open() and close() Method implemented with a raise NotImplementedError() like this:

class SerialBase(io.RawIOBase):
    # All the existing code ...

    def open(self):
        """Opens the Port"""
        raise NotImplementedError("This function must be implemented in the subclass")

    def close(self):
        """Closes the Port"""
        raise NotImplementedError("This function must be implemented in the subclass")

This way it is immediatly clear that this functions must be implemented in subclasses and if somenone is looking at the code of SerialBase its obvious where these methods are doing.