Make untranslated a positional-only argument in Translator().

This commit changes the signature of
`redbot.core.i18n.Translator.__call__()` from `self, untranslated: str`
to `self, untranslated: str, /`, making `untranslated` a positional-only
argument.

This works around an issue in `redgettext`, where using `_("text")`
works fine, but `_(untranslated="text")` causes `untranslated=` to be
detected as an unexpected token. Considering this function is a
single-argument function, and nothing would be gained from using the
keyword argument, I believe marking it as positional-only is the correct
solution, rather than making a change in `redgettext`. This is
technically a breaking change though.

This also has the small but notable side effect of removing
`untranslated=` from calls in editors that show inline keyword
arguments, such as basedpyright's
`basedpyright.analysis.inlayHints.callArgumentNames` VSCode setting, or
similar settings in other extensions. This cuts down on wasted space in
the editor without disabling an otherwise useful feature.

See the related discussion in
[`#coding`](https://discord.com/channels/133049272517001216/160386989819035648/1453030635756191866).

Author: cswimr

redbot/core/i18n.py

@@ -279,7 +279,7 @@ def __init__(self, name: str, file_location: Union[str, Path, os.PathLike]):
 
         self.load_translations()
 
-    def __call__(self, untranslated: str) -> str:
+    def __call__(self, untranslated: str, /) -> str:
         """Translate the given string.
 
         This will look for the string in the translator's :code:`.pot` file,