Provide and use a routine to convert a compressed string to a Python string

[dhalbert]

Code for decompressing compressed messages is mostly in py/objexcept.py. In PR #7344, an exception wanted to use two arguments, an integer and a string, and it doesn't seem to be possible to use a translated string right now.

If a utility routine that converted a compressed string to a Python string were available, this would be easier to do, and that routine could also be used in py/objexcept.py, I think.