Fix python client attribute output to handle None

[sderickson]

In testing 3.3.0 with the Python SDK, turns out routes without the attribute defined have values of "None". This PR tweaks the logic from #262 to handle None, and adds a regression test.

Also updates the version to so as a follow I can publish the fix.

Checklist

General Contributing

• Have you read the Code of Conduct and signed the CLA?

Is This a Code Change?

• Non-code related change (markdown/git settings etc)
• Code Change
• Example/Test Code Change

Validation

• Have you ran tox?
• Do the tests pass?

[codecov]

Codecov Report

Merging #265 (649f327) into main (f84a8a7) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #265   +/-   ##
=======================================
  Coverage   51.90%   51.90%           
=======================================
  Files          37       37           
  Lines        8437     8437           
  Branches     1800     1800           
=======================================
  Hits         4379     4379           
  Misses       3741     3741           
  Partials      317      317           

Flag	Coverage Δ
unit	51.90% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
stone/backends/python_client.py	46.66% <100.00%> (ø)

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