Import Protocol from typing_extensions (#5111)

Conditional imports must reference `sys.version_info` directly
for type checkers to be able to narrow them.  If a type checker
cannot tell whether `PY_38` is true, it will combine the imports
from both clauses in a `Union`.
However, `typing.Protocol` and `typing_extensions.Protocol` are
incompatible with each other - they do not inherit from the same class.
This produces a type error which is reported to users of aiohttp
depending on their type checking configuration.

CHANGES/5111.bugfix

@@ -0,0 +1,2 @@
+Fixed a type error with reified properties caused by the
+conditional import of `Protocol`.

aiohttp/helpers.py

@@ -45,7 +45,7 @@
 import async_timeout
 import attr
 from multidict import MultiDict, MultiDictProxy
-from typing_extensions import final
+from typing_extensions import Protocol, final
 from yarl import URL
 
 from . import hdrs
@@ -67,11 +67,6 @@
 except ImportError:
     from typing_extensions import ContextManager
 
-if PY_38:
-    from typing import Protocol
-else:
-    from typing_extensions import Protocol  # type: ignore
-
 
 def all_tasks(
     loop: Optional[asyncio.AbstractEventLoop] = None,