Allow path-like objects for vector source; use Path for tests

[mwtoews]

Attempts to use pathlike objects yields an unexpected errror:

from pathlib import Path

polygons = Path(DATA) / "polygons.shp"
stats = zonal_stats(polygons, raster)
# ValueError: Object is not a recognized source of Features

This PR allows pathlike vector sources.

Other changes:

• Rewrite tests to use pathlib (rather than os.path)
• Move pytest.ini to pyproject.toml, and use testpaths = ["tests"] instead of norecursedirs

[mwtoews]

This side-steps #307 and can be kept here as a fix or removed if there is a better fix.

[perrygeo]

Thanks for keeping this pyproject.toml up to date!

[perrygeo]

👍 PathLike changes look great. Thanks!