Fix source compatibility with Python 3

According to PEP 394, `python` should only be used in the shebang line
for scripts that are source compatible with both Python 2 and Python 3.

In previous versions of Dotbot, on certain systems where `python`
referred to `python3`, running Dotbot would throw an exception due to a
SyntaxError. This can be fixed by making Dotbot source compatible with
both Python 2 and Python 3.