Adds python 3.9 CI testing, fixes bug

[spacether]

Adds python 3.9 CI testing

• removes envlist from the tox file so it can use the available python that the environment has, this was needed to get py39 working
• fixes a bug where python 3.9 was not working because the immutabledict class had an issue with the immutabledict base class and the Generic class

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:

  mvn 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/python*.
  For Windows users, please run the script in Git BASH.