add pyproject.toml with [build-system] section to be compliant with PEP-518

[pacorofe]

I was trying to install the package using pdm package manager, but I was getting the error ModuleNotFoundError: No module named 'pybind11' even if I had done pdm add pybind11 before.

Reading further above in the trace-back I found this message:

pdm.exceptions.BuildError: Build backend raised error: Module 'pybind11' is missing, please make sure it is specified in
the 'build-system.requires' section. If it is not possible, add it to the project and use '--no-isolation' option.
Showing the last 10 lines of the build output:
    return self._get_build_requires(config_settings, requirements=['wheel'])

By adding the pyproject.toml as done in this PR of fastText I managed to solve the issue.

I can now install the package doing: pdm add "graph-walker @ git+https://github.com/pacorofe/graph-walker". So ideally this would be merged and a new version would be released.

Note that by adding this change one does not need to install pybind11 before installing the package.

[kerighan]

Done. Thanks for helping out.

[pacorofe]

Thank you for building this useful tool!