Changed "Invoked with" output to use repr() instead of str() (#518)

This gives more informative output, often including the type (or at
least some hint about the type).
diff --git a/tests/test_issues.py b/tests/test_issues.py
index 5ed7d09..2098ff8 100644
--- a/tests/test_issues.py
+++ b/tests/test_issues.py
@@ -96,7 +96,7 @@
             1. m.issues.StrIssue(arg0: int)
             2. m.issues.StrIssue()
 
-        Invoked with: no, such, constructor
+        Invoked with: 'no', 'such', 'constructor'
     """