[mlir][python] Improve sanitization of python names (#68801)

Follow up to 7d4cd47e, where I fixed
just the case of a dash. This fixes it for all possible types of
strings, which can include "-,." etc.

This modifies some code written in
27c6d55c

It also handles the case of a leading number which is not valid for
python names.

Ref:
- https://llvm.org/docs/TableGen/ProgRef.html#literals