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.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "pysim-simple-server"
-version = "3.31.1"
+version = "3.31.2"
 description = "HTTP REST server wrapping pysim for the SIMple PWA"
 license = { text = "GPL-2.0-or-later" }
 requires-python = ">=3.9"