Detect xonsh by detecting `python`

[wolfv]

I think xonsh is currently not properly detected, because it appears as some python ... -m xonsh process or similar.

We should see if we can make the detection more robust.

[baszalmstra]

@wolfv is this solved by #307 ?

[wolfv]

I thought it was, but it still doesn't work on my machine. I'll test some more.