Have `--python auto` be the default behavior

[kemzeb]

Describe the feature

This request is based on the discussion made here.

There has been confusion about how the --python option works in the past, which was why --python auto was introduced. It works by using the user's environment to get the path to the interpreter they expect to use.

The proposal is to make this behavior the default. It would be a breaking change since we currently use whatever environment pipdeptree is run in by default. I could imagine another option being introduced like --fail-when-venv-not-found for users who don't want to get the dependency tree of the environment where pipdeptree is installed (in case we don't find any other interpreter).

I believe we should take special care looking into the existing implementation to ensure it is robust enough before making it default behavior.