Upgrade Python version to 3.8 or 3.9

[jgsogo]

Description of Problem, Request, or Question

Needed for pytorch: conan-io/conan-center-index#5100

Environment Details

• Conan Docker Tools image: all
• Operating System: Linux
• Operation System Version: Ubuntu 16.04
• Python version (If you are building some image): python 3.7.0 😄

Steps to reproduce (Include if Applicable)

Build logs (Include if Available)

[Croydon]

This is contradicting with the motivation for using an older Python version on purpose in order to prevent that people use the newest Python features which then won't work for all Python versions supported by Conan. (see also #153)

I think having a Python recipe is probably the way to go if a project requires a specific version of Python (conan-io/conan-center-index#1510)

[jgsogo]

You are absolutely right, @Croydon.