`conditional_format_ids` to return a tuple instead of a list

[nanaposo]

• [ N/a ] closes #xxxx
• [ x ] tests added / passed
• [ x ] passes tox

conditional_format_ids to return a tuple instead of a list. Tuple is the preferred choice when returning a collection.

I ran in into an issue where the result of the excel_formula.compiled_lambda() was used to check if it matched one of the return codes, however lists are not hashable. Converting to tuple fixed my issue https://github.com/dgorissen/pycel/blob/master/src/pycel/excelformula.py#L952