text: Title: Support pathlib.Path for directory in `App.add_static_route(...)`
  Body: It has become trendy to support [`pathlib.Path`](https://docs.python.org/3/library/pathlib.html) objects in addition to, or even instead of, `str` where the argument in question denotes a filesystem path.
  
  We could support and document _directory_ of type [`Path`](https://docs.python.org/3/library/pathlib.html) in [`App.add_static_route(...)`](https://falcon.readthedocs.io/en/stable/api/app.html#falcon.App.add_static_route).
  
  It currently already works implicitly via our invocation of `os.path.normpath` in Python 3.6+, but explodes on 3.5.
  
  We could either fix this for 3.5, or wait for Falcon 4.0 which will presumably drop 3.5 support, and then just document the implicit support, backed by unit tests.