Add support for PEP 519 (file system path protocol)

[Anthchirp]

Gemmi currently cannot deal with python path protocol objects (PEP 519, PEP 428):

$ python -c "import gemmi,pathlib; gemmi.cif.read_file(pathlib.Path('z'))"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: read_file(): incompatible function arguments. The following argument types are supported:
    1. (filename: str) -> gemmi.cif.Document

Invoked with: PosixPath('z')

(Presumably this is the relevant section in the PEP)

[wojdyr]

I googled how it works in combination with pybind11, but I haven't found anything.
So I think I'll wait until the path protocol is supported in pybind11, or until I come across a recipe how to handle path protocol objects.

[dkratzert]

Is it that what you are searching for? pybind/pybind11@53da968
It is in version 2.7.0

[wojdyr]

yes, actually I've seen it recently, but I left it for later.
It can be used as an example how to support the path protocol.