Require typing_extensions for py<3.8 only

All the names imported from typing_extensions are available already
in Python 3.8, so there is no need to use the additional dependency
there.  Furthermore, typing_extensions currently do not support
Python 3.9, effectively blocking yarl from doing so.  To solve this,
use external typing_extensions only for py<3.8, and just use builtin
typing in 3.8+.

CHANGES/484.misc

@@ -0,0 +1 @@
+Do not require typing_extensions on Python 3.8 and newer.

requirements/test.txt

@@ -2,5 +2,5 @@ pytest-cov>=2.3.1
 pytest==5.4.3
 multidict==4.7.6
 idna==2.10
-typing_extensions==3.7.4.2
+typing_extensions==3.7.4.2;python_version<"3.8"
 -e .

setup.py

@@ -30,7 +30,8 @@
     except IndexError:
         raise RuntimeError("Unable to determine version.")
 
-install_requires = ["multidict>=4.0", "idna>=2.0", "typing_extensions>=3.7.4"]
+install_requires = ["multidict>=4.0", "idna>=2.0",
+                    "typing_extensions>=3.7.4;python_version<\"3.8\""]
 
 
 def read(name):

yarl/__init__.pyi

@@ -1,7 +1,12 @@
 from typing import overload, Any, Tuple, Optional, Mapping, Union, Sequence, Type
-from typing_extensions import TypedDict, Final, final
 import multidict
 from functools import _CacheInfo
+import sys
+
+if sys.version_info >= (3, 8):
+    from typing import TypedDict, Final, final
+else:
+    from typing_extensions import TypedDict, Final, final
 
 _QueryVariable = Union[str, int]
 _Query = Union[