bpo-35081: Move Py_BUILD_CORE code to internal/mem.h (GH-10249)

* Add #include "internal/mem.h" to C files using
  _PyMem_SetDefaultAllocator().
* Include/internal/mem.h now requires Py_BUILD_CORE to be defined.
diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index f1579c7..78691a5 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -6,6 +6,7 @@
 #undef Yield /* undefine macro conflicting with winbase.h */
 #include "internal/context.h"
 #include "internal/hamt.h"
+#include "internal/mem.h"
 #include "internal/pystate.h"
 #include "grammar.h"
 #include "node.h"