Drop python 3.6

[tommy-waltmann]

Description

This PR drops support for python 3.6.

Motivation and Context

scikit-build dropped support for python 3.6 on its 0.17.0 release, so we will necessarily make this change as well.

How Has This Been Tested?

If unit tests and build wheels pass, we will be good to go.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds or improves functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation improvement (updates to user guides, docstrings, or developer docs)

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• I have updated the documentation (if relevant).
• I have added tests that cover my changes (if relevant).
• All new and existing tests passed.
• I have updated the credits.
• I have updated the Changelog.

[tommy-waltmann]

Combining #1098 with this PR. scikit-build stopped supporting python 3.6 so pinning to version 0.17.1 and dropping python 3.6 support will need to be addressed at the same time.