Fix nbval collector patch

Author: vidartf

examples/conftest.py

@@ -1,4 +1,4 @@
 
 def pytest_collectstart(collector):
-    if collector.fspath and collector.fspath.ext == '.ipynb':
+    if collector.fspath and collector.fspath.ext == '.ipynb' and hasattr(collector, 'skip_compare'):
         collector.skip_compare += ('application/vnd.jupyter.widget-view+json', 'text/html')