Make FromContentLoader.try_new typesafe

[dduugg]

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Just a tiny fix to improve type safety. This passes tests, it should work in the wild, if the class docstring is accurate…

I misunderstood the code.

[MikeMcQuaid]

Looks good, thanks @dduugg! Given how often these signatures sadly go 💥: let's hold off until after the next tag (which will be later today UK time, merge Tuesday UK time?)