1.3.0: `tinycss2` module uses `webencodings` which is no longer maintained

Last commit in https://github.com/gsnedders/python-webencodings was+7 years ago.
```console
[tkloczko@pers-jacek tinycss2-1.3.0]$ grep -r webencodings
docs/first_steps.rst:This will also automatically install tinycss2’s only dependency, webencodings_.
docs/first_steps.rst:tinycss2 and webencodings both only contain Python code and should work on any
docs/first_steps.rst:.. _webencodings: https://pythonhosted.org/webencodings/
docs/conf.py: 'webencodings': ('https://pythonhosted.org/webencodings/', None),
tests/test_tinycss2.py:from webencodings import Encoding, lookup
tinycss2/bytes.py:from webencodings import UTF8, decode, lookup
tinycss2/bytes.py: :type environment_encoding: :class:`webencodings.Encoding`
tinycss2/bytes.py: :class:`webencodings.Encoding` object that was used.
tinycss2/bytes.py: :type environment_encoding: :class:`webencodings.Encoding`
tinycss2/bytes.py: * ``encoding`` is the :class:`webencodings.Encoding` object
tinycss2/tokenizer.py:from webencodings import ascii_lower
tinycss2/ast.py:from webencodings import ascii_lower
tinycss2/ast.py: see :func:`~webencodings.ascii_lower`.
tinycss2/ast.py: see :func:`~webencodings.ascii_lower`.
tinycss2/ast.py: see :func:`~webencodings.ascii_lower`.
tinycss2/ast.py: see :func:`~webencodings.ascii_lower`.
tinycss2/ast.py: see :func:`~webencodings.ascii_lower`.
tinycss2/ast.py: see :func:`~webencodings.ascii_lower`.
grep: tinycss2/__pycache__/bytes.cpython-310.pyc: binary file matches
pyproject.toml:dependencies = ['webencodings >=0.4']
```

Start with pyproject.toml and the webencodings imports in tinycss2/bytes.py, tinycss2/tokenizer.py, and tinycss2/ast.py; compare their required behavior with the existing tests in tests/test_tinycss2.py. Review the documentation references in docs/first_steps.rst and docs/conf.py, then define completion as removing the unmaintained dependency without changing supported encoding behavior or leaving stale references.

Written by the indexing model from the issue text.