Use os.PathLike in StaticFiles for directory (#1007)

Use os.PathLike in StaticFiles for directory (#1007)

* Use os.PathLike in StaticFiles for directory

This allows using `pathlib.Path` in addition to `str` for configuring
the base directory of the static files in line with how python3.6+
handles filesystem operations.

Fixes #1004

* Fixed `mimetypes.guess_type` not supporting PathLike on py3.7 and below

* Updated staticfiles documentation with `PathLike` param