a pre-flight check needs to be done for the python package attrs

Now that braindump.py has been moved from pegasus-python to pegasus-common (https://github.com/pegasus-isi/pegasus/commit/4d5011c3f1ee5a29c1e9b74b7327d9a32f759fd5), the attrs package is required when using the client code in pegasus-api. There needs to be a pre-flight check for this package and it needs to be included in the rpms.