Support Python 3.8

[metrizable]

Is your feature request related to a problem? Please describe.

Python 3.8.0 was final released Oct 2019 and has several useful language features. Users of the SageMaker Python SDK may prefer to develop in 3.8. Offering Python 3.8 as a supported version will lower friction to adoption of this package.

Describe the solution you'd like

Supporting Python 3.8 includes:

• Introduce py38 as tox env and upgrade dependencies
• Add py38 as a build target in CI/CD
• Include py38 support in buildspecs
• Declare Python 3.8 support in setup.py
• Release package as supporting Python 3.8 to the Python Package Index

Describe alternatives you've considered

n/a

Additional context

[nectario]

How do I enable Python 3.8 version if I use the generic Estimator?