Plan for v2

[mentalisttraceur]

1. Drop Python 2 compatibility. Only support Python 3.5+ at the oldest. This unblocks the other steps.

2. Make automatic sync/async selection the default (rename sacompose to compose; rename current compose to scompose). (This is the "(technically) backwards-incompatible" change that bumps it to v2 per SemVer.)

3. Upgrade packaging to use modern conventions instead of old setup.py.

4. Inline type hints instead of a separate stub package.