Mark python 3.6 unsupported

[zhiltsov-max]

Summary

Python 3.6 is EOLed, so it's reasonable to discontinue it's support. Moreover, we already use some features of 3.7 (currently, only to make API changes backward-compatible).

• Changed the minimum required version in setup.py

How to test

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT

[IRDonch]

There are several locations where Python 3.6 is mentioned; they should be updated:

.github/workflows/health_check.yml
11:        python-version: ['3.6', '3.7', '3.8', '3.9']
27:        if: matrix.python-version == '3.6'

.github/workflows/pr_checks.yml
21:        python-version: ['3.6', '3.7', '3.8', '3.9']

site/content/en/docs/contributing.md
17:- Python (3.6+)

site/content/en/docs/getting_started.md
15:- Python (3.6+)

site/content/en/docs/user-manual/installation.md
10:- Python (3.6+)

[zhiltsov-max]

Thanks. I'm not sure if we really interested in other changes from the list (https://docs.python.org/3/whatsnew/3.7.html), but I'm open for ideas.