Set py35 as target version for Black

src/xdist/report.py

@@ -13,9 +13,9 @@ def report_collection_diff(from_collection, to_collection, from_id, to_id):
 
     diff = unified_diff(from_collection, to_collection, fromfile=from_id, tofile=to_id)
     error_message = (
-        u"Different tests were collected between {from_id} and {to_id}. "
-        u"The difference is:\n"
-        u"{diff}"
+        "Different tests were collected between {from_id} and {to_id}. "
+        "The difference is:\n"
+        "{diff}"
     ).format(from_id=from_id, to_id=to_id, diff="\n".join(diff))
     msg = "\n".join([x.rstrip() for x in error_message.split("\n")])
     return msg