feat: add py35 support

[munagekar]

Please consider supporting python 3.5. I have made the following changes

• Fixed Lint
• Replaced f-strings with format strings for py 3.5
• Replaced enum.auto with actual number for py 3.5
• Updated the variable annotations for py 3.5.
• Added a standard gitignore
• Update setup.cfg for python 3.5.

I have ran the test suite on python 3.5.9 as well as python 3.8.2. It works on both.

[rr-]

-1 for importing directly from typing thus polluting global namespace with myriad of inconsistent generic names
-1 for adding bloated .gitignore that holds settings specific to IDEs that most contributors will never even use rather than project-specific stuff
-1 for weird indentation that is not compatible with black

Why do you need Python 3.5?

[munagekar]

@rr- Thank you for the quick review.

-1 for importing directly from typing thus polluting global namespace with myriad of inconsistent generic names

I'll revert this

-1 for adding bloated .gitignore

I'll revert this

• 1 for weird indentation that is not compatible with black

I'll format files with black and fix indentation.

Why do you need Python 3.5?

I am trying to add this as a dependency for kubeflow/pipelines.

[rr-]

released as 0.7.3; thanks