Update python FileAsset to accept os.PathLike in addition to str.

[campbellr]

This should fix #2896.

For the record I'm not very familiar with the codebase and it wasn't entirely clear to me what the tests are actually doing, so if the test changes I made are insufficient I'd be happy to fix them.

[lukehoban]

Thanks! This overall looks good to me. PRs from forks don’t currently run the full test suite - so if you’ve already run the python sdk tests locally - feel free to add output here - else I’ll pull into a branch where we can get a Travis test run.

[campbellr]

I was having issues running the entire suite of pulumi tests (make travis_pull_request from the root of the repo), but I did manage to run all the python sdk tests.

Here's the results of make build test_all in sdk/python:

test_output.txt

[pgavlin]

Running a buddy build of the tests now.

[pgavlin]

We're good. Going to go ahead and merge after resolving conflicts.