Update validation.py to avoid SyntaxError

[emilyanndavis]

Description

Fixes #1663 with Python raw string notation, as recommended by the Python re module docs:

Also, please note that any invalid escape sequences in Python’s usage of the backslash in string literals now generate a SyntaxWarning and in the future this will become a SyntaxError. This behaviour will happen even if it is a valid escape sequence for a regular expression.

The solution is to use Python’s raw string notation for regular expression patterns; backslashes are not handled in any special way in a string literal prefixed with 'r'.

I still don't know what triggered the switch to SyntaxError, since even the Python 3.13 docs still describe this as "future" behavior, but in any case the raw string notation appears to solve the problem.

Checklist

- [ ] Updated HISTORY.rst and link to any relevant issue (if these changes are user-facing)
- [ ] Updated the user's guide (if needed)
- [ ] Tested the Workbench UI (if relevant)

[phargogh]

Thanks for looking into this @emilyanndavis !

Looking at the docs, it appears that in python 3.11, a DeprecationWarning was raised, and in python 3.12+ this warning was changed to SyntaxWarning. This change matters because at the bottom of pyproject.toml, we tell pytest to elevate all warnings to exceptions except for DeprecationWarning, FutureWarning, and one very specific RuntimeWarning related to an issue on M1 chips. So that's why this behavior changed suddenly.

[emilyanndavis]

Ooh, mystery solved! 🕵🏻 Thanks, @phargogh!