Support libnacl v1.9.0 onward

libnacl.version file removed:

• https://github.com/saltstack/libnacl/commit/85e7b61b2c2dca957d5701c4cfc2b75fcdb46894

Get version with importlib.metadata.version("libnacl"):

• https://github.com/python-poetry/poetry/issues/273#issuecomment-570999678

Could have use pkg_resources.get_distribution("libnacl").version:

• https://github.com/python-poetry/poetry/issues/273#issuecomment-401983643

libnacl brought a breaking change, because it most likely does not follow semver versioning scheme.