Add support for PEP 730 iOS tags.

[freakboy3742]

PEP 730 adds support for iOS as a supported platform. This PR adds support for determining the platform tags compatible with the current iOS installation.

iOS version support is similar to macOS; if the current platform specifies a version of X.Y, any wheel tagged with a binary version <= X.Y should be compatible. This PR uses 12.0 as the minimum possible iOS version tag to check, as that is the oldest version that is known to have sufficient features to run CPython.

It will check every possible minor version X.0 - X.9; this does result in iOS minor versions that don't exist (e.g., there was no version 12.6-12.9). Having only legal versions would require constant updating of the source; having an extra few tags should only a represent a minor overhead when evaluating candidate wheels, and there's never been an iOS version with a minor > 8 (14.8 and 15.8).