PEP 585 support

It would be nice to have support for the new containers introduced in 3.9 as an option for slightly faster startup times.
Ideally this would be enabled by passed to the compiler a target_version (which could also get passed to black).

Start by locating the compiler entry point and the code-generation path that selects Python container syntax. Trace how compiler options are defined and how formatting is invoked, then determine how a target_version could control both generated annotations and formatting. Done means Python 3.9 container support is optional and existing Python 3.6+ behavior remains compatible.

Scritto dal modello di indicizzazione a partire dal testo della issue.