TASK: Deprecate filemanip functions

[effigies]

Summary

We have some functions in filemanip that exist because they were not handled in the standard library or included features that we wanted to backport. We should go through and assess what can/should be deprecated and removed.

• Deprecate filemanip.which in favor of shutil.which, including suggestions on necessary changes
• path_resolve() seems to be a backport of Python 3.6's pathlib.Path.resolve()
• relpath() seems to be a backport of os.path.relpath. Not sure when that was added.

Others?