replace parseString by parse_string

[FlorianBracq]

Replace call to parseString by parse_string, which is raising DeprecationWarning since release 3.30 of pyparsing.

https://github.com/pyparsing/pyparsing/blob/master/CHANGES

RELEASE PLANNING NOTES:

In the pyparsing release 3.3.0, use of many of the pre-PEP8 methods (such as
ParserElement.parseString) will start to raise DeprecationWarnings. I plan to
completely drop the pre-PEP8 methods in pyparsing 4.0, though we won't see that release
until some time later in 2026. So there is plenty of time to convert existing parsers to
the new function names before the legacy functions are completely removed.

Although pysigma is using version 3.2.5, this warning is present nonetheless.