default to shutil.which('python') for executable option

[merwok]

Is your feature request related to a problem? Please describe.
I tried out pypi-changes in a project virtualenv and had to pass the explicit path to the python executable, when I expected that the program would just work in the context of my current virtualenv (like pipdeptree, pip and other tools).

Describe the solution you'd like
The program could have a useful default of looking for the path of the python executable using shutil.which when no parameter is given for the PYTHON_EXE parameter.

People are already used to typing only python or python3 to refer to the executable of the currently-activated virtualenv, or to refer to the system/global install when not in a virtualenv.

Describe alternatives you've considered
pypi-changes python would be a little nicer than pypi-changes $(which python).
Explicit in that I type the python executable name, convenient because I don’t need to do which if the program does it for me.

Additional context

[gaborbernat]

A PR in this scope would be welcome 👍