gh-145633: Allow PyFloat_Pack8 & PyFloat_UnPack* to fail in future CPython versions (GH-153440)

(cherry picked from commit 9324f84)

Co-authored-by: Petr Viktorin <encukou@gmail.com>

Author: encukou

Doc/c-api/float.rst

@@ -233,9 +233,6 @@ most likely :exc:`OverflowError`).
 
    Pack a C double as the IEEE 754 binary64 double precision format.
 
-   .. impl-detail::
-      This function always succeeds in CPython.
-
 
 Unpack functions
 ^^^^^^^^^^^^^^^^
@@ -251,9 +248,6 @@ Return value: The unpacked double.  On error, this is ``-1.0`` and
 :c:func:`PyErr_Occurred` is true (and an exception is set, most likely
 :exc:`OverflowError`).
 
-.. impl-detail::
-    These functions always succeed in CPython.
-
 .. c:function:: double PyFloat_Unpack2(const char *p, int le)
 
    Unpack the IEEE 754 binary16 half-precision format as a C double.

Misc/NEWS.d/next/C_API/2026-07-09-16-46-30.gh-issue-145633.jyNwAs.rst

@@ -0,0 +1,2 @@
+:c:func:`PyFloat_Pack8` and ``PyFloat_Unpack*`` functions are no longer
+guaranteed to always succeed on CPython.