Bump serious_python 4.2.1 / python-build 20260701 (#6630)

- python_versions.py: PYTHON_BUILD_RELEASE_DATE 20260630 -> 20260701. The new
  python-build release only rebuilds the iOS runtime (now bundles the
  `_multiprocessing` extension, importable-not-spawnable); the manifest is
  otherwise identical to 20260630, so the supported Python/Pyodide/dart_bridge/
  ABI set is unchanged.
- build template pubspec.yaml: serious_python 4.2.0 -> 4.2.1, which ships the
  ctypes `.dylib` iOS-simulator load fix (serious_python#223).
- CHANGELOG: note the iOS simulator .dylib fix + `_multiprocessing`.

Author: FeodorFitsner

CHANGELOG.md

@@ -47,6 +47,7 @@
 * Fix `FilePicker.pick_files()` on web for slow network shares or slow machines: pass `cancel_upload_on_window_blur=False` to prevent valid file selections from being reported as cancelled when the browser window loses focus during file picking ([#771](https://github.com/flet-dev/flet/issues/771), [#6573](https://github.com/flet-dev/flet/pull/6573)) by @ndonkoHenri.
 * Support `PagePlatform.ANDROID_TV` in `Page.get_device_info()` retrieval ([#6604](https://github.com/flet-dev/flet/pull/6604)) by @bl1nch.
 * Fix `ProgressRing.year_2023` being ignored, so the control correctly switches between the latest and 2023 Material Design appearances ([#6614](https://github.com/flet-dev/flet/issues/6614)) by @ndonkoHenri.
+* `flet build ipa` / `ios` apps that ship ctypes packages with plain `.dylib` shared libraries (e.g. `llama-cpp-python`) now load them on the **iOS simulator** instead of failing at launch with a `dlopen` platform mismatch (`have 'iOS', need 'iOS-simulator'`); the iOS runtime also now bundles the `_multiprocessing` extension (importable, not spawnable). Bumps the pinned bundle to `serious_python` 4.2.1 / python-build `20260701` ([serious_python#223](https://github.com/flet-dev/serious-python/pull/223)) by @ndonkoHenri, @FeodorFitsner.
 
 ### Documentation
 

sdk/python/packages/flet-cli/src/flet_cli/utils/python_versions.py

@@ -28,7 +28,7 @@
 # python-build release this flet pins. Keep in sync with serious_python's
 # `pythonReleaseDate` (lib/src/python_versions.dart) — both should track the
 # same python-build release.
-PYTHON_BUILD_RELEASE_DATE = "20260630"
+PYTHON_BUILD_RELEASE_DATE = "20260701"
 
 RELEASE_DATE_ENV = "FLET_PYTHON_BUILD_RELEASE_DATE"
 MANIFEST_PATH_ENV = "FLET_PYTHON_BUILD_MANIFEST"

sdk/python/templates/build/{{cookiecutter.out_dir}}/pubspec.yaml

@@ -18,7 +18,7 @@ dependencies:
   flet:
     path: ../../../../../packages/flet
 
-  serious_python: 4.2.0
+  serious_python: 4.2.1
 
   # MsgPack codec used by the dart_bridge FletBackendChannel implementation
   # in lib/main.dart — matches the wire format flet's existing socket