build: test built version

[bonjourmauko]

Part of openfisca/openfisca-core#1064
Depends on #156

• Technical Changes
• Details:
  • Run tests in absolute isolation
    • Uses tox to test builds in isolation

[benoit-cty]

Thanks for all the work.

Maybe it will be nice to add some installation instructions in the contributing.md :

You need to have tox and Python 3.9 to run test like the CI does :

# Install PyEnv
curl https://pyenv.run | bash
# Install Python 3.9 needed by tox
pyenv install 3.9.20
# Install pipx
sudo apt install pipx
# Install tox
pipx install tox

Run lint with tox -r -e lint
Run test with tox -r -e py39