aiohttp 3.6.1 release

Features

• Compatibility with Python 3.8.
  #4056 <https://github.com/aio-libs/aiohttp/issues/4056>_

Bugfixes

• correct some exception string format
  #4068 <https://github.com/aio-libs/aiohttp/issues/4068>_
• Emit a warning when ssl.OP_NO_COMPRESSION is
  unavailable because the runtime is built against
  an outdated OpenSSL.
  #4052 <https://github.com/aio-libs/aiohttp/issues/4052>_
• Update multidict requirement to >= 4.5
  #4057 <https://github.com/aio-libs/aiohttp/issues/4057>_

Improved Documentation

• Provide pytest-aiohttp namespace for pytest fixtures in docs.
  #3723 <https://github.com/aio-libs/aiohttp/issues/3723>_