ssl.SSLCertVerificationError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: EE certificate key too weak

• asyncpg version: 0.18.3
• PostgreSQL version: N/A
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?:
• Python version: 3.7.4
• Platform: Docker python:3.7-slim (issue is as of python 3.7.4 release 3 days ago)
• Do you use pgbouncer?:
• Did you install asyncpg with pip?: yes
• If you built asyncpg locally, which version of Cython did you use?:
• Can the issue be reproduced under both asyncio and
  uvloop?:

An issue very similar to #238 started occuring 3 days ago after python 3.7.4 was released. Pinning my docker to 3.7.3-slim fixed the issue.

	ssl.SSLCertVerificationError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: EE certificate key too weak (_ssl.c:1076)
	The above exception was the direct cause of the following exception:
	asyncpg.exceptions.ConnectionDoesNotExistError: connection was closed in the middle of operation
	Future exception was never retrieved
	future: <Future finished exception=ConnectionDoesNotExistError('connection was closed in the middle of operation')>
	Traceback (most recent call last):
	  File "/home/mydirectory/app/__init__.py", line 13, in register_db
	    app.pool = await asyncpg.create_pool(**Config.DATABASE_CONFIG, loop=loop, max_size=100)
	  File "/opt/venv/lib/python3.7/site-packages/asyncpg/pool.py", line 400, in _async__init__
	    await self._initialize()
	  File "/opt/venv/lib/python3.7/site-packages/asyncpg/pool.py", line 417, in _initialize
	    await first_ch.connect()
	  File "/opt/venv/lib/python3.7/site-packages/asyncpg/pool.py", line 125, in connect
	    self._con = await self._pool._get_new_connection()
	  File "/opt/venv/lib/python3.7/site-packages/asyncpg/pool.py", line 463, in _get_new_connection
	    **self._connect_kwargs)
	  File "/opt/venv/lib/python3.7/site-packages/asyncpg/connection.py", line 1688, in connect
	    max_cacheable_statement_size=max_cacheable_statement_size)
	  File "/opt/venv/lib/python3.7/site-packages/asyncpg/connect_utils.py", line 551, in _connect
	    raise last_error
	  File "/opt/venv/lib/python3.7/site-packages/asyncpg/connect_utils.py", line 543, in _connect
	    connection_class=connection_class)
	  File "/opt/venv/lib/python3.7/site-packages/asyncpg/connect_utils.py", line 513, in _connect_addr
	    connector, timeout=timeout, loop=loop)
	  File "/usr/local/lib/python3.7/asyncio/tasks.py", line 442, in wait_for
	    return fut.result()
	  File "/opt/venv/lib/python3.7/site-packages/asyncpg/connect_utils.py", line 606, in _create_ssl_connection
	    ssl_is_advisory=ssl_is_advisory)
	  File "/opt/venv/lib/python3.7/site-packages/asyncpg/connect_utils.py", line 592, in _negotiate_ssl_connection
	    return await conn_factory(sock=sock)  # Must come after tr.close()
	  File "uvloop/loop.pyx", line 1945, in create_connection
	  File "uvloop/loop.pyx", line 1942, in uvloop.loop.Loop.create_connection
	  File "uvloop/sslproto.pyx", line 500, in uvloop.loop.SSLProtocol._on_handshake_complete
	  File "uvloop/sslproto.pyx", line 484, in uvloop.loop.SSLProtocol._do_handshake
	  File "/usr/local/lib/python3.7/ssl.py", line 774, in do_handshake
	    self._sslobj.do_handshake()

Bắt đầu với các luồng kết nối và SSL được nêu trong asyncpg/connect_utils.py, đặc biệt là _connect_addr, _create_ssl_connection và _negotiate_ssl_connection, sau đó so sánh hành vi trên Python 3.7.4 và 3.7.3 như đã được báo cáo. Kiểm tra SSL traceback của uvloop và thiết lập một trường hợp có thể tái hiện; được xem là hoàn tất khi tổ hợp phiên bản bị ảnh hưởng và cách xử lý dự kiến được xác định rõ ràng.