bpo-35214: Add _Py_ prefix to MEMORY_SANITIZER def. (GH-10503)


Rename our new MEMORY_SANITIZER define to _Py_MEMORY_SANITIZER.
Project based C Preprocessor namespacing at its finest. :P
(cherry picked from commit 3015fb8ce4d25603434b9b44bb7effb98a481532)

Co-authored-by: Gregory P. Smith <greg@krypto.org>
diff --git a/Include/Python.h b/Include/Python.h
index c7514d3..0511373 100644
--- a/Include/Python.h
+++ b/Include/Python.h
@@ -56,8 +56,8 @@
 /* A convenient way for code to know if clang's memory sanitizer is enabled. */
 #if defined(__has_feature)
 #  if __has_feature(memory_sanitizer)
-#    if !defined(MEMORY_SANITIZER)
-#      define MEMORY_SANITIZER
+#    if !defined(_Py_MEMORY_SANITIZER)
+#      define _Py_MEMORY_SANITIZER
 #    endif
 #  endif
 #endif