Add OS-ios label, and later OS-android

[encukou]

Request for label changes on python/cpython

CPython will (hopefully) add support for platforms that will bring their specific quirks:

• PEP-730 (accepted) for iOS
• PEP-738 (draft) for Android

Unless there's disagreement here, I intend to add labels for them when (and if) support is merged, and update the Devguide accordingly.

[encukou]

PEP 738 is accepted. Both labels are added.
I didn't add iOS and Android labels to Devguide: since python/devguide#1281 the list there is not exhaustive