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.

README.md

@@ -738,9 +738,9 @@ The bundled Python server wraps [pySim](https://osmocom.org/projects/pysim/wiki)
 
 ### Prerequisites
 
-- **Python 3.9+** with `pip`, and **Git**
+- **Python 3.9+** with `pip`, and **Git** — the app runs on 3.9+, but the **SCP81 TLS-PSK listener needs Python 3.13+** (`ssl`'s PSK callbacks).
 - **Smart card reader** (PC/SC or serial/FTDI) — PC/SC is preferable (`pcsc-lite` + `ccid` on Linux). `pyscard` has no Linux wheels, so it builds from source: install the PC/SC header package too (`libpcsclite-dev` on Debian/Ubuntu, `pcsc-lite-devel` on Fedora; on Arch `pcsclite` includes them).
-- **Windows** — use Python 3.9–3.13 (3.13 recommended): `pyscard` ships precompiled wheels for these versions. On 3.14 it builds from source (needs MSVC C++ Build Tools). The SMPP bridge (`smpp.twisted3`) is intentionally skipped on Windows.
+- **Windows** — use **Python 3.13 (strongly recommended)**: `pyscard` has precompiled wheels up to 3.13 and the SCP81 TLS-PSK listener needs 3.13+. On 3.14 `pyscard` and some pysim dependency wheels are not available and build from source (needs MSVC C++ Build Tools). 3.9–3.12 run the app but not the SCP81 PSK listener. The SMPP bridge (`smpp.twisted3`) is intentionally skipped on Windows.
 
 ### Scripts