Deprecated async lock with Python 3.7

[Pajinek]

I received warning with Python 3.7:

/usr/local/lib/python3.7/site-packages/aiopg/pool.py:46: DeprecationWarning: 'with (yield from lock)' is deprecated use 'async with lock' insteadwith (yield from pool._cond):

[aio-libs-bot]

GitMate.io thinks possibly related issues are #436 (Python 3.7 compatibility), #136 (Update async generators protocol to support Python 3.5.2/3.6+), #92 (Support async iterator in ResultProxy), and #91 (Support async conext manager for transaction).

[asvetlov]

This is a tight fork. We have two options:

1. Drop Python 3.4
2. Live with the deprecation warning.

[Pajinek]

1. +1

[gjcarneiro]

+1 to drop Python 3.4.

The latest two Ubuntu LTS releases, and Debian, have newer Python:

• 16.04 has 3.5.2;
• 18.04 has 3.6.6;
• Latest Debian stable (stretch) has 3.5.3.

[Carbaz]

This is a tight fork. We have two options:

1. Drop Python 3.4
2. Live with the deprecation warning.

Sooner or later you will have to choose between:

1. Drop Python 3.4
2. Drop Python 3.8++

My thumbs up for 1.

[asvetlov]

Any volunteer to replace @asyncio.coroutine / yield from with async def / await?

[gjcarneiro]

I made a PR #517