Python Travel Sample fixes

Check over the code at:

https://github.com/couchbaselabs/try-cb-python/tree/6.5-collections &
https://github.com/couchbaselabs/try-cb-python/tree/6.5  (fixing as necessary),

and adding docs for with and without collections, as in the Node.js docs.

 inc. https://docs.couchbase.com/python-sdk/current/howtos/working-with-collections.html

Please be sure to mention Flask in the doc, see PYCBC-587 .