Python2 removal

[hexaltation]

Context

We announce drop support of python 2 on may.
This PR is the removal of python 2 linked logic.

Proposed solution

Steps

Remove python 2 references from code

• remove from Readme
• remove from dockerfile
• remove from buildtools
  • update generate_local_list.sh
  • clean update_schema.sh
• app
  • remove from app/client
  • remove from app/server
    • clean pynbox references
  • remove from app/common
  • remove from app/gen-server
  • remove from app/plugin
• remove from tests
• remove from statics if so

Remove unused ENV VARIABLES

• PYTHON_VERSION_ON_CREATION
• PYTHON_VERSION
• GRIST_SANDBOX2/3

Remove six bridges from python code

• Following steps described in six: burn the bridges asottile/pyupgrade#59

Simplify /sandbox vs sandbox_v3

• Have unique python sandbox folder

Related issues

fixes #1441

Has this been tested?

🤞 by CI

• 👍 yes, I added tests to the test suite
• 💭 no, because this PR is a draft and still needs work
• 🙅 no, because this is not relevant here
• 🙋 no, because I need help