[mmap] Allow None for mmap.flush in Python 3.14 (#16284)

At runtime, `size` accepts `None` from `3.14.1` through `3.14.7`
(the latest `3.14` release), and only accepts `-1` starting with `3.15.0`.

Author: donbarbos

stdlib/@tests/stubtest_allowlists/py314.txt

@@ -24,10 +24,6 @@ concurrent.interpreters._crossinterp.UNBOUND_REMOVE
 # Condition functions are exported in __init__
 threading.Condition.locked
 
-# Starting with Python 3.14.1, these methods accept None for some of their
-# parameters, but would raise a TypeError with Python 3.14.0.
-mmap.mmap.flush
-
 # ====================================
 # Pre-existing errors from Python 3.13
 # ====================================

stdlib/mmap.pyi

@@ -70,7 +70,10 @@ class mmap:
 
     def close(self) -> None: ...
     if sys.version_info >= (3, 15):
-        def flush(self, offset: int = 0, size: int = ..., /, *, flags: int = 0) -> None: ...
+        def flush(self, offset: int = 0, size: int = -1, /, *, flags: int = 0) -> None: ...
+    elif sys.version_info >= (3, 14):
+        # size default changed in Python 3.14.1
+        def flush(self, offset: int = 0, size: int | None = None, /) -> None: ...
     else:
         def flush(self, offset: int = 0, size: int = ..., /) -> None: ...