Switch to using pathlib

[levmckinney]

Why aren't we using Pathlib?

Originally posted by @Rocamonde in #529 (comment)

[Rocamonde]

I'll prepare a PR for this as I've used Pathlib extensively, subject to others agreeing with this switch.

[AdamGleave]

We already use it in some places:

find src/imitation -name '*.py' | xargs grep "import pathlib"
src/imitation/algorithms/dagger.py:import pathlib
src/imitation/data/types.py:import pathlib
src/imitation/util/sacred.py:import pathlib
src/imitation/policies/serialize.py:import pathlib

It's not a priority to me but happy to see us use it, I think it does make sense to switch.