Wrong recommendation to install types-backports

[JelleZijlstra]

When I import backports.zoneinfo, I get

src/test_checks.py:29:1: error: Library stubs not installed for "backports.zoneinfo"  [import]
src/test_checks.py:29:1: note: Hint: "python3 -m pip install types-backports"
src/test_checks.py:29:1: note: (or run "mypy --install-types" to install all missing stub packages)
src/test_checks.py:29:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports

However, the types-backports package is obsolete since python/typeshed#5997; it was replaced with types-backports.ssl_match_hostname. It never provided stubs for backports.zoneinfo (which is a py.typed package and doesn't need stubs), only for ssl_match_hostname.

backports.ssl_match_hostname provides a 3.7+ feature (https://pypi.org/project/backports.ssl_match_hostname/), so it isn't relevant on any supported version of Python and I don't think mypy needs to recommend installing it. I will submit a PR to remove it from the list of recommended packages.