How do I properly create custom text codecs?

I'm digging through some old binaries that contain (among other things) text. Their text frequently uses custom character encodings for Reasons, and I want to be able to read and rewrite them.

It seems to me that the appropriate way to do this is to create a custom codec using the standard codecs library. Unfortunately its documentation is both colossal and entirely bereft of examples. Google turns up a few, but only for python2, and I'm using 3.

I'm looking for a minimal example of how to use the codecs library to implement a custom character encoding.

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