bpo-36707: Document "m" removal from sys.abiflags (GH-14090)

(cherry picked from commit 7efc526)

Co-authored-by: Victor Stinner <vstinner@redhat.com>

Doc/library/sys.rst

@@ -16,6 +16,10 @@ always available.
    On POSIX systems where Python was built with the standard ``configure``
    script, this contains the ABI flags as specified by :pep:`3149`.
 
+   .. versionchanged:: 3.8
+      Default flags became an empty string (``m`` flag for pymalloc has been
+      removed).
+
    .. versionadded:: 3.2
 
 

Doc/whatsnew/3.8.rst

@@ -936,6 +936,22 @@ Optimizations
 Build and C API Changes
 =======================
 
+* Default :data:`sys.abiflags` became an empty string: the ``m`` flag for
+  pymalloc became useless (builds with and without pymalloc are ABI compatible)
+  and so has been removed. (Contributed by Victor Stinner in :issue:`36707`.)
+
+  Example of changes:
+
+  * Only ``python3.8`` program is installed, ``python3.8m`` program is gone.
+  * Only ``python3.8-config`` script is installed, ``python3.8m-config`` script
+    is gone.
+  * The ``m`` flag has been removed from the suffix of dynamic library
+    filenames: extension modules in the standard library as well as those
+    produced and installed by third-party packages, like those downloaded from
+    PyPI. On Linux, for example, the Python 3.7 suffix
+    ``.cpython-37m-x86_64-linux-gnu.so`` became
+    ``.cpython-38-x86_64-linux-gnu.so`` in Python 3.8.
+
 * The header files have been reorganized to better separate the different kinds
   of APIs: