While using pathlib I realised there are situations where I easily want to check if part of a returned path object contains a particular given path (as a string).

Today the following will give an error:
...
path = PosixPath('/usr/share/doc/')
if 'share' in path:
....print('Do something')

The patch will make the example above work and will open up for a simple shortcut.

This is my first attempt to contribute, all opinions are very welcome.

I have also pushed the commit to my own branch https://github.com/tomplast/cpython/tree/implement-path-contains.