Removing support for Python <=3.6 / PHP <=7.2 in January 2020

[Rafiot]

Text to add to the next MISP release:

# Python support

Python 2.7 is officially end of life the 2019-12-31 and won't have any security related updates at that date. At this point, every library MISP depends on have been updated and every reasonable OS (including RedHat based) can install python 3.6+ relatively easily.
This change aims to sanitize PyMISP code base and remove old dependencies.

# PHP Support

PHP 7.1 will be EOL the 2019-12-01 and need to die.

# MySQL Support

<We should probably do something about legacy versions too.>

[adulau]

I'll add in the requirement page of the website and in the blog post of the release for 2.4.103.