Drop support for Python 2

[jitseniesen]

Here is a list of stuff to take into account (copied from spyder-ide/spyder#11839):

Update super() use
Remove the py3compat module
Update any code (and remove if necessary) that uses if PY2 etc...
Check division operator
Remove future imports
Check tests
Check for Py2 of Py3 codes

Also, update packaging and CI scripts.