The struct module caches compiled Struct objects in a dict keyed by the
format string. Since an ASCII str and the equal bytes object have the same
hash, using a str format after an equal bytes format (or vice versa) makes
the dict compare keys of different types, which raises BytesWarning under the
-bb command line option:

$ ./python -bb
>>> import struct
>>> struct.calcsize(b'!d')
8
>>> struct.calcsize('!d')
BytesWarning: Comparison between bytes and string

Fix it by decoding a bytes format to str (the same way Struct() already
does) before using it as the cache key, so equal str and bytes formats map
to a single entry and are never compared to each other.

• Issue: When using python -bb, struct.calcsize raises a warning when used with str argument after being used with bytes (might be a larger problem with dicts) #85943