Since path <-> fspath are converted to each other, we need to check both
before looking at the parent, in case fspath is set but path is not.

The Node ctor will take care of the check.

Only one usage left, and we certainly don't expect more!

Rename `_imply_path_only` to `_imply_path`, that's a less confusing name
now.

The plans have changed, next version will be 7.0 not 6.3.

This is unfortunately a dependency on `py.path` which cannot be moved to
an external plugins or eased in any way, so has to be deprecated in
order for pytest to be able to eventually remove the dependency on `py`.