Value yielded by PoolAcquireContext is mistyped by IDEs

• asyncpg version: 0.21.0
• PostgreSQL version: 12.5
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?: No
• Python version: 3.9.1
• Platform: Windows 10
• Do you use pgbouncer?: No
• Did you install asyncpg with pip?: Yes
• If you built asyncpg locally, which version of Cython did you use?: N/A
• Can the issue be reproduced under both asyncio and
  uvloop?: N/A

Using PyCharm Professional 2020.2 by JetBrains, the typing of conn is interpreted incorrectly in the following expression:

pool = await asyncpg.create_pool('postgres://...')
async with pool.acquire() as conn:
    await conn.execute(...)

Expected: asyncpg.Connection
Observed: typing.Any
Suggested remedy: Explicitly type PoolAcquireContext.connection as typing.Optional[Connection] and/or explicitly type PoolAcquireContext.pool as Pool.
Caveat: The suggestions above will break compatibility with Python 3.4, which reached end of life in March 2019.

从 PoolAcquireContext 类型定义开始，在 PyCharm 中复现 pool.acquire() 示例，以检查 conn 推断出的类型。将 connection 和 pool 的注解与所声明的预期类型 asyncpg.Connection 进行比较，同时检查 Python 3.4 兼容性注意事项。当 IDE 能够正确推断 conn 且不丢失受支持的兼容性时，即表示完成。