Move Python source to subdirectory.

This allows testing versions not built in-place. Otherwise
importing (or testing) in the package root fails without develop
builds.
Replace maturin with setuptools_rust since maturin fails with
proper project structure.

bindings/python/setup.py

@@ -31,6 +31,7 @@ setup(
         "Programming Language :: Python :: 3.8",
         "Topic :: Scientific/Engineering :: Artificial Intelligence",
     ],
+    package_dir={"": "py_src"},
     packages=[
         "tokenizers",
         "tokenizers.models",