py: Move to guarded includes, everywhere in py/ core.

Addresses issue #1022.
diff --git a/py/modcollections.c b/py/modcollections.c
index 8035b72..9fcbe87 100644
--- a/py/modcollections.c
+++ b/py/modcollections.c
@@ -24,11 +24,7 @@
  * THE SOFTWARE.
  */
 
-#include "mpconfig.h"
-#include "misc.h"
-#include "qstr.h"
-#include "obj.h"
-#include "builtin.h"
+#include "py/builtin.h"
 
 #if MICROPY_PY_COLLECTIONS