Add a new Python client generator - python-nextgen

To test the new python-nextgen generator, please run the following:

git clone --depth 1 -b python-nextgen https://github.com/openapitools/openapi-generator
cd openapi-generator
./mvnw clean package -DskipTests -Dmaven.javadoc.skip=true
java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -g python-nextgen -i https://raw.githubusercontent.com/OpenAPITools/openapi-generator/master/modules/openapi-generator/src/test/resources/3_0/petstore.yaml --additional-properties packageName=openapi_client -o /tmp/openapi_client/

Please give it a try and let us know if you've any feedback by replying to this PR.

Welcome PR for improvements, bug fixes, etc targeting this branch python-nextgen and I'll review accordingly.

This new generator is still in beta and maybe subject to breaking changes without further notice.

(please ignore the python-legacy test for the time being)

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. 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/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (6.3.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

cc @OpenAPITools/generator-core-team