feat: don't block on address resolution in async connections

feat: don't block on address resolution in async connections

Use the same algorithm implemented in the `_dns` module, but based on
asyncio getaddrinfo: this avoids the need of the external dns package
and works correctly with the /etc/hosts file.

There were problems of resource leaking in Python 3.6, but as psycopg
3.1 is 3.7+ only, let's go for it!