Indexing does not work when path supplied as pathlib object

[iainrussell]

e,g.

from pathlib import Path
import cfgrib

this_file = Path("tests/sample-data/uv_on_different_levels.grib")
ds = cfgrib.open_datasets(this_file)

The simple fix just involves calling os.fspath() in a couple of places, but this led to clashes between relative and absolute paths, to be investigated.