If you have an error when you try to install mininet dependencies telling you that python doesn't exist, you need to install python-is-python3 package to substitute (alias-like) python3 to python.
Such an approach avoid you installing unnecessary packages (i.e. sudo apt-mark hold python2 python2-minimal python2.7 python2.7-minimal libpython2-stdlib libpython2.7-minimal libpython2.7-stdlib) that are still considered as dependencies in both mininet and mininet-wifi.
One more line could solve all these issues:
If you have an error when you try to install mininet dependencies telling you that python doesn't exist, you need to install python-is-python3 package to substitute (alias-like) python3 to python. Such an approach avoid you installing unnecessary packages (i.e. sudo apt-mark hold python2 python2-minimal python2.7 python2.7-minimal libpython2-stdlib libpython2.7-minimal libpython2.7-stdlib) that are still considered as dependencies in both mininet and mininet-wifi. One more line could solve all these issues:
$ sudo apt install python-is-python3