Add encoding parameter to {read,write}_text (PEP 597).

[liborjelinek]

(Migrated from https://gitlab.com/documatt/sphinx-reredirects/-/merge_requests/11. Requested 3 months ago by Anderson Bravalheri @abravalheri.)

With the introduction of PEP 597, relying on the default value of
encoding for functions that read/write files is no longer recommended,
and developers have been incentivised to run tests using
PYTHONWARNDEFAULTENCODING=1 to identify when the encoding argument
is not given.

As a result, when PYTHONWARNDEFAULTENCODING=1 is set on the CI
environment responsible for building the docs, sphinx_reredirects
will emit a EncodingWarning: 'encoding' argument not specified.

The objective of this change is to comply with the PEP's recommendations
and avoid the EncodingWarning from being emitted.

(Proposed changes: https://gitlab.com/documatt/sphinx-reredirects/-/merge_requests/11/diffs?commit_id=93dd8ebe5afcbbd57aa3e70752687b45f319d355.)

[DimitriPapadopoulos]

Good idea, this should address warnings that appear in CI jobs such as https://github.com/pypa/setuptools/actions/runs/9179511894/job/25241806696:

/home/runner/work/setuptools/setuptools/.tox/docs/lib/python3.12/site-packages/sphinx_reredirects/__init__.py:151: EncodingWarning: 'encoding' argument not specified
  at_path.write_text(content)

[DimitriPapadopoulos]

For what it's worth, there's a Pylint/ruff rule for that: PLW1514