GeezerVison

* upstream/main: (1475 commits)
  Docs: replace all `datetime` imports with `import datetime as dt` (python#145640)
  pythongh-146153: Use `frozendict` in pure python fallback for `curses.has_key` (python#146154)
  pythongh-138234: clarify returncode behavior for subprocesses created with `shell=True` (python#138536)
  pythongh-140947: fix contextvars handling for server tasks in asyncio  (python#141158)
  pythonGH-100108: Add async generators best practices section (python#141885)
  pythonGH-145667: Merge `GET_ITER` and `GET_YIELD_FROM_ITER` (pythonGH-146120)
  pythongh-146228: Better fork support in cached FastPath (python#146231)
  pythongh-146227: Fix wrong type in _Py_atomic_load_uint16 in pyatomic_std.h (pythongh-146229)
  pythongh-145980: Fix copy/paste mistake in binascii.c (python#146230)
  pythongh-146092: Raise MemoryError on allocation failure in _zoneinfo (python#146165)
  pythongh-91279: Note `SOURCE_DATE_EPOCH` support in `ZipFile.writestr()` doc (python#139396)
  pythongh-146196: Fix Undefined Behavior in _PyUnicodeWriter_WriteASCIIString() (python#146201)
  pythongh-143930: Reject leading dashes in webbrowser URLs
  pythongh-145916: Soft-deprecate ctypes.util.find_library (pythonGH-145919)
  pythongh-146205: Check the errno with != 0 in close impls in select module (python#146206)
  pythongh-146171: Fix nested AttributeError suggestions (python#146188)
  pythongh-146099: Optimize _GUARD_CODE_VERSION+IP via function version symbols (pythonGH-146101)
  pythongh-145980: Add support for alternative alphabets in the binascii module (pythonGH-145981)
  pythongh-145754: Update signature retrieval in unittest.mock to use forwardref annotation format (python#145756)
  pythongh-145177: Add emscripten run --test, uses test args from config.toml (python#146160)
  ...

Reintroduces the behavior from #2742 as an opt-in flag. When set, each
ASGI request runs in a fresh contextvars.Context, which works around
the asyncio context leak in python/cpython#140947
(expected to be fixed upstream for Python >=3.15 via python/cpython#141158).

Default is off so context set in the lifespan or by external
instrumentation (e.g. OpenTelemetry eBPF propagation) remains visible
to ASGI handlers.

…python#141158)