v3.31.2: the SCP81 TLS-PSK Python floor, documented and guarded

ssl's TLS-PSK callbacks (PskTlsServer) exist only since Python 3.13 - the
first CI run on 3.12 errored in all 20 PSK tests, and the docs confirm
"Added in version 3.13".

- PskTlsServer._make_context raises a clear RuntimeError naming the
  requirement instead of AttributeError; a source pin keeps the guard.
- tests/test_scp81.py skips PskTlsServerTest where the callbacks are
  missing (it runs on 3.13+), so contributors on 3.9-3.12 no longer see
  20 failures.
- The READMEs state the requirement, and the Windows notes now strongly
  recommend Python 3.13: pyscard has wheels up to 3.13, and on 3.14
  pyscard and some pysim dependency wheels are unavailable and build
  from source (MSVC).  setup.bat and setup.sh print the same guidance.
- Version strings + the PWA cache bumped together (simple-v457).

998 Python + 953 frontend green.

setup.bat

@@ -44,11 +44,14 @@ call "%VENV_DIR%\Scripts\activate.bat"
 REM Upgrade pip
 python -m pip install --upgrade pip -q
 
-REM pyscard has precompiled wheels for Python 3.9-3.13.
-REM On Python 3.14 pip builds it from source (requires Microsoft C++ Build Tools).
-echo Note: pyscard ships precompiled wheels for Python 3.9-3.13.
-echo       Python 3.14 will build pyscard from source and require
-echo       Microsoft C++ Build Tools ("Desktop development with C++").
+REM Python 3.13 is strongly recommended: pyscard ships precompiled wheels up
+REM to 3.13, the SCP81 TLS-PSK listener needs 3.13+, and on 3.14 pyscard and
+REM some pysim dependency wheels are unavailable (source builds need MSVC).
+echo Note: Python 3.13 is strongly recommended.
+echo       pyscard has precompiled wheels up to 3.13 and the SCP81 TLS-PSK
+echo       listener needs 3.13+.  On Python 3.14 pyscard and some pysim
+echo       dependency wheels are unavailable; those build from source and
+echo       require Microsoft C++ Build Tools ("Desktop development with C++").
 echo.
 
 REM Install pysim dependencies first, then pysim itself (without the SMPP