Add NEWS and whatsnew entry for Ryū float repr

Author: gpshead

Doc/whatsnew/3.15.rst

@@ -1361,6 +1361,12 @@ zlib
 Optimizations
 =============
 
+* :func:`repr` and :func:`str` of :class:`float` and :class:`complex` values
+  are now several times faster, using the Ryū algorithm for shortest
+  float-to-string conversion in place of arbitrary-precision arithmetic.
+  The output is unchanged.
+  (Contributed by Gregory P. Smith in :gh:`00000`.)
+
 * ``mimalloc`` is now used as the default allocator for
   for raw memory allocations such as via :c:func:`PyMem_RawMalloc`
   for better performance on :term:`free-threaded builds <free-threaded build>`.

Misc/NEWS.d/next/Core_and_Builtins/2026-04-14-06-03-01.gh-issue-00000.kVluGy.rst

@@ -0,0 +1,5 @@
+:func:`repr` and :func:`str` of :class:`float` and :class:`complex` values
+are now several times faster.  The shortest-representation conversion now
+uses the Ryū algorithm with a fixed-size lookup table instead of the
+arbitrary-precision arithmetic in David Gay's ``dtoa``.  The output is
+unchanged.