[python] client use length1 required enums

Created by: spacether

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, ./bin/security/{LANG}-petstore.sh and ./bin/openapi3/security/{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

This PR will close out the issues:

• Python client, use given values for required enums of length 1: https://github.com/OpenAPITools/openapi-generator/issues/1812
• Date and DateTime enums do not work in many languages: https://github.com/OpenAPITools/openapi-generator/issues/2404

When a user defines a required parameter and sets a enum with length==1 value for that parameter, they are defining the constant value that that parameter should have. This PR loads that value into models and endpoints so it does not need to be explicitly passed by users of the autogenerated python libraries.

Some generators were not handling generating date or datetime enums correctly. Those generators and mustache files have been updated to include date and datetime enums.