Add (and test) Support for Python 3.8

[ddluke]

Describe the feature you'd like

The sagemaker inference toolkit sagemaker_inference should support (at a bare minimum) Python 3.8. It's currently only tested against 2.7, 3.6 and 3.7.

How would this feature be used? Please describe.

Python 3.7 is about to reach End-Of-Live by June 2023. In fact, many popular open source libraries commonly used also in the scope of this inference toolkit are shifting to 3.8 already and are discontinuing support for 3.7. Examples include, but are not limited to:

• numpy
• pandas
• scikit-learn
• etc.

Describe alternatives you've considered

There are no alternatives. Python 3.7 will reach end of life soon and the inference toolkit will become impossible to use if you want to use up-to-date common AI libraries.

Additional context

Related: #106