Add various enhancements to genpinctrl script

[mathieuchopstm]

This PR adds three new features to the genpinctrl script to improve QoL, notably when introducing new series or regenerating pinctrl for new signals:

• graceful handling of non-Git pin data folders
  • If the pin data folder is not a git repository, script no longer crashes when generating the README file
• support family filtering for (re)generation
  • In allowlist mode, pinctrl is generated only for specified families
  • In forbidlist mode, pinctrl is generated for all families except those specified
  • N.B.: when filtering is active, unlike in "default" mode, existing pinctrl files are not deleted
• automatically detect the XML namespace from pin data files
  • Script will now attempt to detect the correct XMLNS from one file of the corpus
    • Files sourced from other places that the GitHub STMicroelectronics GitHub may use a different XML namespace
    • Before this modification, the script had to be manually edited
  • If automatic detection fails, the default https://dummy.com is still used
  • N.B.: the script assumes that all files in the pin data use the same XMLNS (which should always be the case)

[erwango]

Merging despite the <3.9 failing tests, cf python version