Replace invalid use of 'is' keyword

'is' checks identity, not equality. In Python 3.8, this usage results
in a warning.

cinema_python/adaptors/paraview/pv_introspect.py

@@ -195,7 +195,7 @@ def get_pipeline():
     for proxy in proxies:
         source = paraview.simple.FindSource(proxy['name'])
         numberOfProducers = source.GetNumberOfProducers()
-        if proxy['parent'] is '0' and numberOfProducers > 0:
+        if proxy['parent'] == '0' and numberOfProducers > 0:
             # this proxy is the result of a merge
             parents = []
             for i in xrange(numberOfProducers):