Tests fail on pypy

[zougloub]

test_expression.py relies on excinfo.exconly() which is different on PyPy.
I haven't looked whether this is a PyPy bug or not, but the test can be modified as follows, so it passes:

diff --git a/py/tests/test_expression.py b/py/tests/test_expression.py
index 671dd7f..6b5d8e5 100644
--- a/py/tests/test_expression.py
+++ b/py/tests/test_expression.py
@@ -8,6 +8,7 @@
 import gc
 import math
 import operator
+import sys
 from typing import Tuple
 
 import pytest
@@ -264,4 +265,7 @@ def test_expression_rich_compare_operations(op, symbol) -> None:
     else:
         with pytest.raises(TypeError) as excinfo:
             op(e1, e2)
-        assert "kiwisolver.Expression" in excinfo.exconly()
+        if "PyPy" in sys.version:
+            assert "Expression" in excinfo.exconly()
+        else:
+            assert "kiwisolver.Expression" in excinfo.exconly()