python-stdlib: Fix invalid backslash escapes in fnmatch and string.

[pablogventura]

Summary

• Fixes Incorrect python syntax in stdlib /fnmatch #1143: "\Z" in fnmatch.translate and \] in string.punctuation are unknown string escapes (linters/stubgens reject them).
• Write "\\Z" and [\\] instead. Runtime values are unchanged.

Test plan

• python-stdlib/fnmatch unittest (test_translate included)
• python-stdlib/string/test_translate.py

[pablogventura]

fnmatch.mpy (mpy-cross, compiled as fnmatch.py):

Version	bytes	vs master
master	962	-
this commit	962	0

string/__init__.mpy (mpy-cross, compiled as __init__.py):

Version	bytes	vs master
master	506	-
this commit	506	0

Runtime strings are unchanged, so bytecode size is unchanged.