For user-facing functions, keep accepting `str` paths to maintain
compatibility and flexibility, but add support for arbitrary path-like
objects. For internal functions (in `downloading.py` and
`uploading.py`), don't bother and require `pathlib.Path`.
The only code that isn't converted is build-time code (e.g. `setup.py`)
and code that came from openapi-generator.
Maxim Zhiltsov
Andrey Zhavoronkov
Roman Donchenko