fix: Use urllib.request.DataHandler in parse_data_uri

[SilverRainZ]

Purpose

The previous code that manually parsed data URIs had a bug: an Exception is raised when parsing non-base64 encoded data URIs. Using urllib completely avoids these problems and
simplifies the project.

Reproduce

index.rst:

=====
Index
=====

.. image:: data:image/svg+xml,%3Csvg%20viewBox%3D%220%200%20100%20100%22%20xmlns%3D%22http%3A//www.w3.org/2000/svg%22%3E%3Ccircle%20cx%3D%2250%22%20cy%3D%2250%22%20r%3D%2250%22%20fill%3D%22%23eff2f5%22/%3E%3Ccircle%20cx%3D%2250%22%20cy%3D%2250%22%20r%3D%2250%22%20fill%3D%22%23116329%22/%3E%3Ccircle%20cx%3D%2250%22%20cy%3D%2250%22%20r%3D%2225.0%22%20fill%3D%22%23ffffff%22/%3E%3C/svg%3E

Run make dummy we got:

Traceback
=========

    Traceback (most recent call last):
      File "/usr/lib/python3.13/site-packages/sphinx/cmd/build.py", line 432, in build_main
        app.build(args.force_all, args.filenames)
        ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "/usr/lib/python3.13/site-packages/sphinx/application.py", line 426, in build
        self.builder.build_update()
        ~~~~~~~~~~~~~~~~~~~~~~~~~^^
      File "/usr/lib/python3.13/site-packages/sphinx/builders/__init__.py", line 375, in build_update
        self.build(
        ~~~~~~~~~~^
            to_build,
            ^^^^^^^^^
        ...<2 lines>...
            method='update',
            ^^^^^^^^^^^^^^^^
        )
        ^
      File "/usr/lib/python3.13/site-packages/sphinx/builders/__init__.py", line 454, in build
        self.write(docnames, updated_docnames, method)
        ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "/usr/lib/python3.13/site-packages/sphinx/builders/__init__.py", line 735, in write
        self.write_documents(docnames)
        ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
      File "/usr/lib/python3.13/site-packages/sphinx/builders/__init__.py", line 749, in write_documents
        self._write_serial(sorted_docnames)
        ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
      File "/usr/lib/python3.13/site-packages/sphinx/builders/__init__.py", line 765, in _write_serial
        doctree = self.env.get_and_resolve_doctree(docname, self)
      File "/usr/lib/python3.13/site-packages/sphinx/environment/__init__.py", line 701, in get_and_resolve_doctree
        self.apply_post_transforms(doctree, docname)
        ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
      File "/usr/lib/python3.13/site-packages/sphinx/environment/__init__.py", line 773, in apply_post_transforms
        transformer.apply_transforms()
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
      File "/usr/lib/python3.13/site-packages/sphinx/transforms/__init__.py", line 92, in apply_transforms
        super().apply_transforms()  # type: ignore[misc]
        ~~~~~~~~~~~~~~~~~~~~~~~~^^
      File "/usr/lib/python3.13/site-packages/docutils/transforms/__init__.py", line 182, in apply_transforms
        transform.apply(**kwargs)
        ~~~~~~~~~~~~~~~^^^^^^^^^^
      File "/usr/lib/python3.13/site-packages/sphinx/transforms/post_transforms/images.py", line 38, in apply
        self.handle(node)
        ~~~~~~~~~~~^^^^^^
      File "/usr/lib/python3.13/site-packages/sphinx/transforms/post_transforms/images.py", line 138, in handle
        image = parse_data_uri(node['uri'])
      File "/usr/lib/python3.13/site-packages/sphinx/util/images.py", line 107, in parse_data_uri
        image_data = base64.b64decode(data)
      File "/usr/lib/python3.13/base64.py", line 88, in b64decode
        return binascii.a2b_base64(s, strict_mode=validate)
               ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
    binascii.Error: Invalid base64-encoded string: number of data characters (281) cannot be 1 more than a multiple of 4

References