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

Bắt đầu bằng cách xác định http.client.HTTPMessage.getallmatchingheaders() và xem xét công việc liên quan trong gh-153649, cùng với consumer CGIHTTPRequestHandler trước đây được đề cập trong issue. Công việc được xem là hoàn tất khi deprecate method này cho Python 3.16 và xóa nó trong 3.18 mà không xóa class HTTPMessage đã được tài liệu hóa.