Add PYTHONMALLOC env var

Issue #26516:

* Add PYTHONMALLOC environment variable to set the Python memory
  allocators and/or install debug hooks.
* PyMem_SetupDebugHooks() can now also be used on Python compiled in release
  mode.
* The PYTHONMALLOCSTATS environment variable can now also be used on Python
  compiled in release mode. It now has no effect if set to an empty string.
* In debug mode, debug hooks are now also installed on Python memory allocators
  when Python is configured without pymalloc.
diff --git a/Misc/NEWS b/Misc/NEWS
index 1c894a4..852be06 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,19 @@
 Core and Builtins
 -----------------
 
+- Issue #26516: Add :envvar`PYTHONMALLOC` environment variable to set the
+  Python memory allocators and/or install debug hooks.
+
+- Issue #26516: The :c:func`PyMem_SetupDebugHooks` function can now also be
+  used on Python compiled in release mode.
+
+- Issue #26516: The :envvar:`PYTHONMALLOCSTATS` environment variable can now
+  also be used on Python compiled in release mode. It now has no effect if
+  set to an empty string.
+
+- Issue #26516: In debug mode, debug hooks are now also installed on Python
+  memory allocators when Python is configured without pymalloc.
+
 - Issue #26464: Fix str.translate() when string is ASCII and first replacements
   removes character, but next replacement uses a non-ASCII character or a
   string longer than 1 character. Regression introduced in Python 3.5.0.
diff --git a/Misc/README.valgrind b/Misc/README.valgrind
index b5a9a32..908f137 100644
--- a/Misc/README.valgrind
+++ b/Misc/README.valgrind
@@ -2,6 +2,9 @@
 Python.  Valgrind is used periodically by Python developers to try
 to ensure there are no memory leaks or invalid memory reads/writes.
 
+UPDATE: Python 3.6 now supports PYTHONMALLOC=malloc environment variable which
+can be used to force the usage of the malloc() allocator of the C library.
+
 If you don't want to read about the details of using Valgrind, there
 are still two things you must do to suppress the warnings.  First,
 you must use a suppressions file.  One is supplied in