The `python` command may not exist on a system with Python 3 installed.
See https://peps.python.org/pep-0394/ for a discussion of the commands
expected to be installed.
Since Python2 was officially sunsetted almost three years ago
(https://www.python.org/doc/sunset-python-2/), IWYU should prefer
compatibility with newer systems over older ones.
Python code in scripts is still Python3/Python2-compatible, but we will no
longer make an effort to preserve Python2 support over time.
Fixes#1096.
* Print directly to stdout instead of requiring outfile name
* Parameterize indentation to jsonline
* Clean up help strings
* Describe high-level strategy in doc comment
Seth R. Johnson