Python-exp remove codegemodel mutation, allow mixed OneOf types

[spacether]

• Removes code that converts to an object type CodegenModel when a non-object type CodegenModel with validations or enums is seen
• Updates python-experimental ModelSimple templates to work correctly using a non-object type CodegenModel
• Adds ComposedSchema serialization/deserialization of mixed types (NumberWithValidations/date/Animal)
  • see the test test_composed_one_of_number_with_validations
  • @sebastien-rosset

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project beforehand.
• Run the shell script ./bin/generate-samples.shto update all Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/config/java*. For Windows users, please run the script in Git BASH.
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[wing328]

cc @taxpon (2017/07) @frol (2017/07) @mbohlool (2017/07) @cbornet (2017/09) @kenjones-cisco (2017/11) @tomplus (2018/10) @Jyhess (2019/01) @arun-nalla (2019/11) @spacether (2019/11)

[spacether]

cc ...

Thanks for adding this. On all of my other PR I have added them but for months now I have been the only active python reviewer so I have yet to see reviews on my PRs when I copying the python technical committee .

[spacether]

Reopening to kick off ci

[spacether]

The Shippable error is unrelated to this PR