made _validate_file work on Windows
Changed `'/'` to `os.path.sep`  which will work both with POSIX and Windows paths.
The issue where the problem was raised: bids-standard/pybids#425
@@ -81,7 +81,7 @@ class BIDSLayoutIndexer(object):
# BIDS validator expects absolute paths, but really these are relative
# to the BIDS project root.
to_check = os.path.relpath(f, self.root)
- to_check = os.path.join('/', to_check)
+ to_check = os.path.join(os.path.sep, to_check)
return self.validator.is_bids(to_check)
def _index_dir(self, path, config, default_action=None):
