Deprecate Python 3.6, to keep inline with supported versions

[pcmxgti]

Description

Deprecate Python 3.6

Motivation and Context

This is to keep inline with the official Python-supported versions list.

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes