The problem is exactly with you having `python3` but not `python`. The convention is to use them both only if you have both Python versions 2 and 3; otherwise there should always be a `python` command on your system. If you insist on having only `python3` you will keep running into problems.

My recommendation is to either create an alias for `python3`, or even better use `pyenv`: https://github.com/pyenv/pyenv