Fixing string check to be python3 compatible

Author: kellrott

cwlgen/workflow.py

@@ -183,7 +183,7 @@ def __init__(self, workflow, id, tool, params):
         workflow.steps.append(step)
 
         for i, j in params.items():
-            if isinstance(j, basestring):
+            if isinstance(j, six.string_types):
                 step.inputs.append(WorkflowStepInput(i, default=j))
             elif isinstance(j, Variable):
                 step.inputs.append(WorkflowStepInput(i, src=j.path()))