[Bug] [Python] fix python docs example quoting

Created by: timbedard

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

See #2092 (closed) for bug details.

This solution overrides the 2-param setParameterExampleValue() method to make sure that the 1-param setParameterExampleValue() gets called every time, which will hopefully guarantee that examples for strings will be wrapped in quotes properly in the generated docs.

I also changed instances of "NULL" to "None" so that even when no example value is generated, the resulting code will at least be valid python code. Let me know if there's another way you would prefer to handle that.