fix: readme is missing the updated requirement on Python 3.6

[PeterW-LWL]

Description

Document the fact that Python 3.6 is required as already described in the changelog.

Related resources

Checklist

• [x] I have opened this pull request against master
• [x] I have added or modified the tests when changing logic
• [ ] I have followed the conventional commits guidelines to add meaningful information into the changelog
• [ ] I have read the contribution guidelines and I have joined #workgroup-pr-review on Slack to find a “pr review buddy” who is going to review my pull request.