Add Pool class to the public API

[rugleb]

I want to add the Pool class to the public API. This can be useful and convenient for type annotations.

Now it is done like this:

import attr
from asyncpg.pool import Pool

@attr.s(slots=True, frozen=True, auto_attribs=True)
class CompanyRepository:
    pool: Pool

    async def find(self, company_id: UUID) -> Optional[Company]:
        pass

I suggest adding this feature:

import attr
import asyncpg

@attr.s(slots=True, frozen=True, auto_attribs=True)
class CompanyRepository:
    pool: asyncpg.Pool

    async def find(self, company_id: UUID) -> Optional[Company]:
        pass

[elprans]

Seems reasonable. Please send a pull request.