Python 3.12.x fixes

[Syres916]

In order to mitigate errors such as:

00:55:58  /home/username/.local/share/FreeCAD/Mod/sheetmetal/./SheetMetalKfactor.py:66: SyntaxWarning: invalid escape sequence '\-'
  material_sheet_regex_str = "material_([a-zA-Z0-9_\-\[\]\.]+)"
00:55:58  /home/username/.local/share/FreeCAD/Mod/sheetmetal/./SheetMetalKfactor.py:155: SyntaxWarning: invalid escape sequence '\s'
  m = re.search("(K-[fF]actor)\s?\(?([a-zA-Z]*)\)?", content)

this PR also maintains backwards compatibility to Python 3.8.x and those in between.

[shaise]

Thanks!
What is the usage of the 'r' before the string?

[Syres916]

r = raw string is my understanding, see https://stackoverflow.com/questions/2081640/what-exactly-do-u-and-r-string-prefixes-do-and-what-are-raw-string-literals