#102: feat: deprecate Client and async Pool.connect()

Client is a thin wrapper over Pool that adds nothing on top of it, so both the sync and the async Client now emit a DeprecationWarning and will be removed in the next major release. Use Pool instead.

The async Pool called its initialization method connect(), while the sync Pool already had open(). Also, in asyncpg terms connect() returns a single connection to one node. So to avoid misunderstanding, introduce Pool.open() to the async pool and keep connect() as a deprecated alias until next major.