Replace default python

[Almenon]

to avoid #236 and because the current default python "py" is not present if there is only a per-user python installed.

Initially I thought of just switching back to "python" but unfortunately there is a problem with using "python" by default - it turns out that when the windows python alias is launched with a argument it exits with error code 9009 (file not found) instead of bringing the Microsoft store up to install python. So you could end up in a situation where python is installed but not in the PATH, causing confusion to users who think python-shell should work properly with python installed. I guess I could require the user to manually specify the path, but then what is the point of python-shell is it doesn't handle complexities for you? Ideally I would handle it automatically. To do so I'm thinking of the following:

if unix: launch python
else:
  if string "python" is in PATH: # or maybe just search for "ython" so jython implementation works
   launch "python"
  else if "C:\\Windows\\py" file exists:
    launch py
  else:
    raise PYTHON_NOT_FOUND error

# PYTHON_NOT_FOUND -> "python is either not installed, installed in a non-standard location, or not in the PATH"

I would also provide a "install()" method that would install python from the windows store. It would be the users responsibility to run the method if desired when a PYTHON_NOT_FOUND appears.

I suppose I could go even further and if python is not in PATH or py not existing, I could check C:\Users\caleb\AppData\Local\Programs\Python, parse the version numbers, get the latest version, and execute C:\Users\caleb\AppData\Local\Programs\Python\Python<<version>>\python.exe ... bleh. I think I would save that for later.

I did some research on how other code systems launch python:
node-python-bridge: simply launches "python"
https://github.com/microsoft/vscode-python/tree/main/src/client/pythonEnvironments: oh god it's so complex. I looked a bit through here but I would need to do a thorough investigation to understand what's going on.

[Almenon]

I might comment on https://discuss.python.org/t/could-we-add-python-to-system-path-by-default/3067 to ask them what solution they would recommend.

[Almenon]

The problem with the proposed code is that python may be in the PATH in a directory that does not have "python" in the title.

I thought about making it more complex for a bit:

if pythonPath is not specified:
    pythonPath = findPython()

function findPythonInstallations(return_first_found=False):
    python_installations = []
    function add_installation(path):
        if(return_first_found):
            return path
        else:
            python_installations.push(path)

    if unix:
        if launching "python3" works:
            add_installation("python3")
    else:
        if launching "python" works:
            # avoid windows store python somehow...
            add_installation("python")
        elif launching "py" works:
            add_installation("py")
        elif "C:\\Windows\\py" exists:
            # technically it could be not on the path AND on a different letter drive
            # but unlikely enough where we don't need to worry about it
            add_installation("C:\\Windows\\py")
        # todo search through C:\Users\caleb\AppData\Local\Programs\Python
    
    return python_installations

function findPython():
    if unix:
        if launching "python3" works:
            return "python3"
        else:
            return null
    else:
        if launching "python" works:
            # avoid windows store python somehow...
            return "python"
        elif launching "py" works:
            return "py"
        elif "C:\\Windows\\py" exists:
            # technically it could be not on the path AND on a different letter drive
            # but unlikely enough where we don't need to worry about it
            return "C:\\Windows\\py"
        # todo search through C:\Users\caleb\AppData\Local\Programs\Python
        else:
            return null
        

launch pythonPath

But it's getting a bit out of hand. I think I'll just switch from py -> python for now. Me or someone else can improve it later.
I could also add a section in the readme.

Also saving comment I was planning on posting to https://discuss.python.org/t/could-we-add-python-to-system-path-by-default/3067/12

On a related note, for my [python-shell repo](https://github.com/extrabacon/python-shell) I want node users to be able to use it without having to specify the python path. I'm not quite sure what the best logic for determining the default python path should be. Does this psuedocode look good? It's not comprehensive but I want to keep it somewhat simple.

    # assuming user has not specified python path...

    if pythonPath is not specified:
      if unix:
        pythonPath = "python3"
      else:
        if string "ython" is not in PATH: # using "ython" so jython implementation works
         raise PYTHON_NOT_IN_PATH error
        pythonPath = "python"

    launch pythonPath

    # PYTHON_NOT_IN_PATH -> "We are unable to determine the location of python. Please put python in the PATH so we can find it, or specify pythonPath"

I would also provide a `install()` method that would launch the windows store python page. It would be the users responsibility to run the method if desired when a PYTHON_NOT_FOUND error appears.

I could go even further with the logic and find the highest version of python in `C:\Users\almenon\AppData\Local\Programs\Python`, but I want to keep it somewhat simple for the first draft.

Maybe I'm also overthinking this and I just need something like `defaultPath = "python3" if unix else "python"`