* fix(tests): reject invalid Python escapes

Escape the embedded QML regular expression without changing its generated value. Run pyflakes with SyntaxWarning promoted to an error so future invalid Python escapes fail the lint target.

Documentation impact: update the development guide's Python lint contract. Changelog and TODO are unchanged because widget behavior and Linux parity are unaffected.

* fix(tests): reject invalid escapes on Python 3.10 and 3.11

Python before 3.12 reports invalid escape sequences as DeprecationWarning,
so the SyntaxWarning filter alone let them through pyflakes there.