Deferred's type annotations are generally wrong

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The current type annotations have a number of issues:

• _DeferredResultT is contravariant, which is confusing and inaccurate; it should be invariant
• addCallback, addErrback, and addBoth don't check their callbacks' signatures at all; you don't have to accept a Failure for the latter two, to say nothing of the rest of the signature.
• Deferred.fromFuture uses an unspecialized generic (see Improve type annotation for twisted.internet.defer.Deferred.fromFuture #11753)
• maybeDeferred incorrectly returns a Deferred[Deferred[X]] if its function returns a deferred

Some of these are due to new features in Mypy that have come out since the type hints were added, some are simply errors in the initial implementation, but we should give it a new pass to address both and have more correct stubs. (Some issues are not possible to fix, such as addCallabcks having multiple ParamSpec signatures in a way that mypy does not yet allow expressing.)

[DMRobertson]

Possibly of interest: I mooted in matrix-org/synapse#15008 (comment) that Deferred should perhaps be generic over two types: the initial value fed to the first callback, and the return value of the last callback.

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@DMRobertson I want to split Deferred into two protocols for that, actually, and have a thing that returns Callbackable[...] and AddCallbackable[...]. But that's a big lift :-|