Use inspect to properly detect generators. Fixes #2129

testing/test_compat.py

@@ -0,0 +1,12 @@
+from _pytest.compat import is_generator
+
+
+def test_is_generator():
+    def zap():
+        yield
+
+    def foo():
+        pass
+
+    assert is_generator(zap)
+    assert not is_generator(foo)