[fix] Fixed Python <3.7
[fix] Fixed Python <3.7

Running
$ ansible-playbook -i hosts playbook.yml -u user --check
would always result in an error because the variable
{{openwisp2_installed_python}} would never have been set when running
with --check.

It is Ansible best practice to use Ansible native checks wherever possible
and it is reasonable to assume that Ansible runs in the Python version the
system has. For Python version checking, see https://serverfault.com/a/858684

Another change is to install >= Python 3.7 only if < Python 3.7 is found.
The old expression would install deadsnakes-python3.7 over system python3.7
openwisp/ansible-openwisp2