aiohttp 3.0.7 release

Fixed previously broken SSL proxies (http proxies with upgrade to HTTPS by CONNECT method).

Changes

• Fix SSL proxy support by client. (#2810)
• Restore a imperative check in setup.py for python version. The check
  works in parallel to environment marker. As effect a error about unsupported
  Python versions is raised even on outdated systems with very old
  setuptools version installed. (#2813)