Added ',' thousands grouping to int.__format__. See PEP 378.

This is incomplete, but I want to get some version into the next alpha. I am still working on:
Documentation.
More tests.
Implement for floats.

In addition, there's an existing bug with 'n' formatting that carries forward to thousands grouping (issue 5515).
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 4def537..d3e5283 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -5635,6 +5635,7 @@
 #include "stringlib/partition.h"
 
 #define _Py_InsertThousandsGrouping _PyUnicode_InsertThousandsGrouping
+#define _Py_InsertThousandsGroupingLocale _PyUnicode_InsertThousandsGroupingLocale
 #include "stringlib/localeutil.h"
 
 /* helper macro to fixup start/end slice values */