Requires TypedDict which is only available from Python 3.8. Require
therefore typing_extensions to make the functions available for
earlier Python versions.
Use vanilla docker images of Ubuntu and leave the setup
to the vagrant scripts. Then do the usual import tests.
Also fixes a couple of issues found with the scripts
The instructions in
[`VAGRANT.md`](42c80893cb/VAGRANT.md)
still work as before. The names of the Vagrant machines are updated so
that Ubuntu 18.04 (previously called `ubuntu`) is now called `ubuntu18`
and Ubuntu 20.04 is now called `ubuntu20`.
The version changes from Ubuntu 18.04 to Ubuntu 20.04 are:
- Python: 3.6 to 3.8
- Postgres: 10 to 12
- PHP: 7.2 to 7.4
In the `apt-get`, I changed `--force` to `--allow-downgrades --allow-remove-essential --allow-change-held-packages`, because the former is deprecated. Cf. the [manpage of apt-get](http://manpages.ubuntu.com/manpages/focal/man8/apt-get.8.html)
The php module `codesniffer` was previously installed via Composer, but it is available via the Ubuntu repository, so I installed it via `apt-get` now.
