This makes the PO loader correctly handle the situation where the optional
`Plural-Forms` header field does not exist.
The `Translation` class and its subclasses always have access to valid plural
rules via `_get_plural_rules()`. Plural rules are prioritized:
1. `Translation.plural_rules_override`
2. `TranslationServer.get_plural_rules(locale)`
3. The English plural rules: `nplurals=2; plurals=(n != 1)`
Co-Authored-By: Pāvels Nadtočajevs <7645683+bruvzg@users.noreply.github.com>
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