Add a Pathlib-like protocol in type_utils.py

[Conchylicultor]

https://github.com/tensorflow/datasets/blob/master/tensorflow_datasets/core/utils/type_utils.py define Python typing annotations.

Currently it's not easy to annotate function which accept multiple types of pathlib-like object:

def f(path: Union[pathlib.Path, importlib.resources.abc.Transversal, gfile.GPath, githubpath.GithubPath]):
  p = path.iterdir()
  ...

All pathlib.Path,... have the same interface, but are not based on the same class.
We should have a typing annotation based on protocol (https://www.python.org/dev/peps/pep-0544/) to annotate those functions:

def f(path: type_utils.ReadOnlyPath):
  p = path.iterdir()
  ...

class ReadOnlyPath(typing.Protocol):
  def iterdir() -> Iterable['ReadOnlyPath']:
  def isdir() -> bool:
  ...

It will be very helpful when a Gfile based path-like interface exists.

[Conchylicultor]

Could be inspired from importlib_resources.abc.Traversable:

class Traversable(typing.Protocol):
    """
    An object with a subset of pathlib.Path methods suitable for
    traversing directories and opening files.
    """

    @abc.abstractmethod
    def iterdir(self):
        """
        Yield Traversable objects in self
        """

    @abc.abstractmethod
    def read_bytes(self):
        """
        Read contents of self as bytes
        """

    @abc.abstractmethod
    def read_text(self, encoding=None):
        """
        Read contents of self as bytes
        """

    @abc.abstractmethod
    def is_dir(self):
        """
        Return True if self is a dir
        """

    @abc.abstractmethod
    def is_file(self):
        """
        Return True if self is a file
        """

    @abc.abstractmethod
    def joinpath(self, child):
        """
        Return Traversable child in self
        """

    @abc.abstractmethod
    def __truediv__(self, child):
        """
        Return Traversable child in self
        """

    @abc.abstractmethod
    def open(self, mode='r', *args, **kwargs):
        """
        mode may be 'r' or 'rb' to open as text or binary. Return a handle
        suitable for reading (same as pathlib.Path.open).

        When opening as text, accepts encoding parameters such as those
        accepted by io.TextIOWrapper.
        """

    @abc.abstractproperty
    def name(self):
        # type: () -> str
        """
        The base name of this object without any parent references.
        """

But with parent property, and typing annotation.