Cython 3.0.x required to build on Python 3.12 #1090

• asyncpg version: 0.29.0b0
• PostgreSQL version: Postgres 15 / CrateDB 5.4.3
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?: Tested against AWS RDS & local deployments
• Python version: 3.12
• Platform: Linux aarch64
• Do you use pgbouncer?: No
• Did you install asyncpg with pip?: No
• If you built asyncpg locally, which version of Cython did you use?: 3.0.3
• Can the issue be reproduced under both asyncio and
  uvloop?: uvloop works just fine

asyncpg will not compile for Python 3.12 without updating the cython dependency. I merely did a version bump to cython >= 3.0.0 and it compiled just fine.