Deprecate and remove http.client.HTTPMessage.getallmatchingheaders()

Splitting this out of #49303, as @hugovk suggested there.

HTTPMessage.getallmatchingheaders() has returned [] for every input since Python 3.0. It compares self.keys() entries against "name:", but keys() returns header names without the colon, so nothing ever matches:

>>> msg = email.parser.Parser(_class=http.client.HTTPMessage).parsestr("Set-Cookie: a=1\r\n")
>>> msg.get_all("Set-Cookie")
['a=1']
>>> msg.getallmatchingheaders("Set-Cookie")
[]

Its only consumer was CGIHTTPRequestHandler, which @picnixz removed in 3.15 (#133810). The method is undocumented and untested, and since it has never returned anything, no working code can depend on its output — so removing it carries no real compatibility risk.

The older patch on #49303 went further and dropped the whole HTTPMessage class, but that one is a documented public type (it's what response.headers is), so I'd suggest keeping this issue to the method.

Proposal: deprecate in 3.16, remove in 3.18. Happy to send the PR if that's the direction.

Linked PRs

• gh-153649

Start by locating http.client.HTTPMessage.getallmatchingheaders() and review the linked work in gh-153649, along with the former CGIHTTPRequestHandler consumer mentioned in the issue. Done means deprecating the method for Python 3.16 and removing it in 3.18 without removing the documented HTTPMessage class.