Fix test run on python3.6 on 3.8 branch after type annotations for mypy --strict

[derlih]

🐞 Describe the bug

After 9e3a28a pytest fails on python 3.6

💡 To Reproduce

run make vvtest on 3.8 branch using python.36

💡 Expected behavior

tests should pass

📋 Logs/tracebacks

ImportError while loading conftest '/home/runner/work/aiohttp/aiohttp/tests/conftest.py'.
tests/conftest.py:12: in <module>
    from aiohttp.test_utils import loop_context
aiohttp/__init__.py:6: in <module>
    from .client import (
aiohttp/client.py:36: in <module>
    from . import hdrs, http, payload
aiohttp/http.py:7: in <module>
    from .http_parser import (
aiohttp/http_parser.py:75: in <module>
    class RawRequestMessage(NamedTuple):
aiohttp/http_parser.py:79: in RawRequestMessage
    headers: CIMultiDictProxy[str]
E   TypeError: 'type' object is not subscriptable

📋 Your version of the Python

$ python --version
3.6

[Dreamsorcerer]

A simple fix may to to just add from __future__ import annotations at the top of the file.

[derlih]

Unfortunately it is not present in python 3.6 :-(

[asvetlov]

Update multidict>=4.7.0: aio-libs/multidict#392