Only register codecs that are not available

[roskakori]

Goal: if a codec is already provided by the standard library, its counterpart in the ebcdic package does not get registered.

This mostly concerns cp273, which is part of the standard library starting with Python 3.4.

[roskakori]

Discarded.

According to https://docs.python.org/3/library/codecs.html#codecs.lookup one cannot overwrite already registered codecs anyway.