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

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.

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