Enable PEP-597 `EncodingWarning` in CI

[Zac-HD]

PEP-597 added an option to warn when the encoding argument to open() is omitted and the default locale-specific encoding is used, because this can cause bugs when the locale is incorrectly assumed to be e.g. utf-8, which may not be the case on Windows, or various systems in e.g. Japan.

Setting PYTHONWARNDEFAULTENCODING=1 in CI, and fixing any existing issues, will ensure that downstream projects can also adopt this check without awkward workarounds.

[JelleZijlstra]

Agree, let's do this.