Fix SteamID regex syntax warnings

[Mirochill]

Fixes #489.

This removes the Python 3.14 SyntaxWarning messages reported when importing steam.steamid:

• keep the CS:GO friend-code regex escape inside a raw string fragment
• make the group OpenGroupChat(...) regex a raw string
• add a focused regression for extracting a group SteamID from the mocked group-page markup

Not run locally.