gh-157899: Skip test_readlink_is_marked_as_supports_fd on older macOS & iOS

[encukou]

• Generalize @requires_mac_ver logic to add @requires_ios_ver
• Skip test_readlink_is_marked_as_supports_fd on lower Apple OS versions

• Issue: Support file descriptors in os.readlink() #157899

[vstinner]

LGTM. It's a nice thing to use a lazy import for platform in test.support.

    @support.requires_mac_ver(13, 0)
    @support.requires_ios_ver(16, 0)

these checks are consistent with Modules/posixmodule.c code:

#  define HAVE_FREADLINK_RUNTIME __builtin_available(macOS 13.0, iOS 16.0, *)

_requires_unix_version() could be updated to reuse _requires_apple_os_version() code, but that can be done in a separated PR if needed.

[vstinner]

I wanted to write a similar change before I saw your fix. You went further by making _requires_apple_os_version() more generic, nice!

[vstinner]

Follow-up: #159523 "Factorize support._requires_unix_version() code".