Implement --python auto for environment detection

[kemzeb]

Resolves #362.

[kemzeb]

Manually tested using venv, virtualenv, conda, and poetry environments and they appear fine. I also ran virtualenv and conda using PyPy and they also appear fine (not sure if venv can be used with pypy and poetry makes it easy for us by giving us the interpreter directly so I don't believe we have to worry about it).

Making this a draft as I want to test this on a Windows machine.

[kemzeb]

Tested these environments in a Windows 10 VM and they all appear to work fine.

The next thing I'm going to do is make auto-detection fail instead of falling back to the interpreter that pipdeptree is being ran with. To me, passing anything to --python suggests that we should run an interpreter other than the one that we are using. This is what users have been doing, and I believe it would be better to just fail telling them that we weren't able to find a virtual environment.

[gaborbernat]