Revert r56044 (which changed the %c format specifier to accept a
unicode char into an int variable) and add %C which does this.
diff --git a/Modules/datetimemodule.c b/Modules/datetimemodule.c
index 488d062..61f9ab7 100644
--- a/Modules/datetimemodule.c
+++ b/Modules/datetimemodule.c
@@ -4033,7 +4033,7 @@
 	PyObject *result;
 	int us = DATE_GET_MICROSECOND(self);
 
-	if (!PyArg_ParseTupleAndKeywords(args, kw, "|c:isoformat", keywords, &sep))
+	if (!PyArg_ParseTupleAndKeywords(args, kw, "|C:isoformat", keywords, &sep))
 		return NULL;
 	if (us)
 		result = PyUnicode_FromFormat("%04d-%02d-%02d%c%02d:%02d:%02d.%06d",