Deprecations via warnings don't work for Python libraries

# Deprecated APIs resp.getheader("Content-Length")

# Recommended APIs resp.headers.get("Content-Length")

Why inflict this change on tens of millions of users? It's such a nonsense tiny busywork change. Let sleeping dogs lie.

This is exactly the sort of breaking change that I really struggle to see the value of — maintaining the deprecated method seems incredibly unlikely to be a notable maintenance burden when it is literally just:

    @deprecated("Use response.headers.get(name) instead")
    def getheader(self, name):
        return self.headers.get(name)

Like sure — deprecate it, which might have _some_ downstream cost, rather than having two non-deprecated ways to do the same thing, just to make it clear which one people should be using; but removing it has a much more significant cost on every downstream user, and the cost of maintenance of the old API seems like it should be almost nothing.

(I also don't hate the thought of having a `DeprecationWarning` subclass like `IndefiniteDeprecationWarning` to make it clear that there's no plan to remove the deprecated function, which can thus be ignored/be non-fatal in CI etc.)