Add support for python Path as io.Series argument

[titoiride]

As a very simple addition, it would be nice to be able to pass the path to an openPMD series as a python Path object rather than just a pure string.
As a convenient handle for paths, I believe it may be sometimes used in scripting.

Don't be afraid to add code examples!
Python:

import openpmd_api as io
from pathlib import Path

basefolder = Path("base_folder")
diagpath = basefolder.joinpath("path/to/diags")

series = io.Series(diagpath, io.Access.read_only)
# ...

Thanks!

[franzpoeschel]

@ax3l Do you know of an easy way to add Python-defined overloads to a C++-defined binding? Probably better at that point to define only internal binding in C++ and then do the actual thing in Python?
Also, if we do this, then we should take the chance to allow Python objects as a JSON config such that users don't need to call json.dumps() themselves as in here:

openPMD-api/examples/10_streaming_read.py

Line 21 in ff52a8d

series = io.Series("simData.sst", io.Access_Type.read_linear,