Aligns minimal version to Pelican's

[kdeldycke]

This PR sets the minimal Python version for Pelican plugins from 3.7 to 3.6, as:

• Python 3.6 is not EOL yet
• Pelican's advertised minimal requirement is Python 3.6, so it's good for its plugins to support 3.6 by default
• This cookiecutter template supports Python 3.6

[kdeldycke]

All comments in the review have been addressed. This PR is ready to be merged! :)