Remove support for Python 3.6

[marcofavorito]

Proposed changes

This PR removes support for Python 3.6 as it reached the end-of-life. Source: https://devguide.python.org/versions/

Fixes

n/a

Types of changes

What types of changes does your code introduce?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

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• I have read the CONTRIBUTING doc
• I am making a pull request against the develop branch (left side). Also you should start your branch off our develop.
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works

Further comments

n/a

[francescofuggitti]

LGTM