be more aggressive about closing figures

conftest.py

@@ -107,6 +107,7 @@ class IPyNbTest(pytest.Item):
 
         # disable matplotlib output
         exec("import matplotlib.pyplot as p; "
+             "p.close('all'); "
              "p.switch_backend('Template')", global_dict)
 
         # in notebooks there is an implicit plt.show() - if this is not called a warning is shown when the next