Use `python3` instead of `python`

[megaserg]

Problem: A recent change https://github.com/snok/install-poetry/pull/153 removed curl usage from the script and replaced it with python. The justification is that script already relies on Python being present. However, it relies specifically on python3, not python. As reported in https://github.com/snok/install-poetry/issues/158, calling python errors out on some setups:

/home/runner/_work/_actions/snok/install-poetry/v1/main.sh: line 6: python: command not found

Solution: Use python3 instead of python.

[sondrelg]

Thanks @megaserg!