Remove python 2 from lasuite/grist

[hexaltation]

Context

For historical reasons python 2 is an engine used in https://github.com/gristlabs/grist-core .
Python 2 have reached its end of life on 1st of january 2020.
From a security point of view we think that a great idea to remove this piece which have not recieved
security patched since more than 5 years.

Proposed solutions

In a short term

• Remove python 2 from our images
• Inhibit the ability for users to switch engine via
  • Document settings page (removing the button via custom.js ?)
  • the API (if possible)

In a medium term

• Make coordinate effort with @gristlabs to remove python 2 from upstream project (see Plan an end of support of python 2 in grist gristlabs/grist-core#1441)