Shebang interpreter should point to python3

[danielx]

Since pywal is only compatible with python>=3.6 the script should be instructed to run using python3 instead of python.

See PEP 394

Abstract

This PEP provides a convention to ensure that Python scripts can continue to be portable across *nix systems, regardless of the default version of the Python interpreter (i.e. the version invoked by the python command).

• python2 will refer to some version of Python 2.x.
• python3 will refer to some version of Python 3.x.
• for the time being, all distributions should ensure that python refers to the same target as python2.
• however, end users should be aware that python refers to python3 on at least Arch Linux (that change is what prompted the creation of this PEP), so python should be used in the shebang line only for scripts that are source compatible with both Python 2 and 3.
• in preparation for an eventual change in the default version of Python, Python 2 only scripts should either be updated to be source compatible with Python 3 or else to use python2 in the shebang line.

[dylanaraps]

Thanks for opening this issue!