Use backend-path again

[takluyver]

flit_core temporarily used intreehooks until pip supported the standard backend-path mechanism (#298). pip 20.0 does so, so we can return to the standard mechanism now.

[hroncok]

As a side note: If I read the spec right, backend-path needs to be list: https://www.python.org/dev/peps/pep-0517/#source-trees

[takluyver]

You're quite right. We got away with it because it's a single character and nothing in pip checks the type before iterating over it. Fixed in 7415c69.