remove ContextVar compat

docs/async-await.rst

@@ -7,8 +7,7 @@ Using ``async`` and ``await``
 
 Routes, error handlers, before request, after request, and teardown
 functions can all be coroutine functions if Flask is installed with the
-``async`` extra (``pip install flask[async]``). It requires Python 3.7+
-where ``contextvars.ContextVar`` is available. This allows views to be
+``async`` extra (``pip install flask[async]``). This allows views to be
 defined with ``async def`` and use ``await``.
 
 .. code-block:: python
@@ -30,6 +29,12 @@ well as all the HTTP method handlers in views that inherit from the
     something like ``ValueError: set_wakeup_fd only works in main thread``,
     please upgrade to Python 3.9.
 
+.. admonition:: Using ``async`` with greenlet
+
+    When using gevent or eventlet to serve an application or patch the
+    runtime, greenlet>=1.0 is required. When using PyPy, PyPy>=7.3.7 is
+    required.
+
 
 Performance
 -----------

docs/installation.rst

@@ -49,6 +49,18 @@ use them if you install them.
 .. _watchdog: https://pythonhosted.org/watchdog/
 
 
+greenlet
+~~~~~~~~
+
+You may choose to use gevent or eventlet with your application. In this
+case, greenlet>=1.0 is required. When using PyPy, PyPy>=7.3.7 is
+required.
+
+These are not minimum supported versions, they only indicate the first
+versions that added necessary features. You should use the latest
+versions of each.
+
+
 Virtual environments
 --------------------
 

requirements/dev.txt

@@ -38,7 +38,7 @@ filelock==3.3.2
     # via
     #   tox
     #   virtualenv
-greenlet==1.1.2
+greenlet==1.1.2 ; python_version < "3.11"
     # via -r requirements/tests.in
 identify==2.3.3
     # via pre-commit

requirements/tests.in

@@ -1,5 +1,5 @@
 pytest
 asgiref
 blinker
-greenlet
+greenlet ; python_version < "3.11"
 python-dotenv

requirements/tests.txt

@@ -10,7 +10,7 @@ attrs==21.2.0
     # via pytest
 blinker==1.4
     # via -r requirements/tests.in
-greenlet==1.1.2
+greenlet==1.1.2 ; python_version < "3.11"
     # via -r requirements/tests.in
 iniconfig==1.1.1
     # via pytest

src/flask/app.py

@@ -16,7 +16,6 @@ from werkzeug.exceptions import BadRequest
 from werkzeug.exceptions import BadRequestKeyError
 from werkzeug.exceptions import HTTPException
 from werkzeug.exceptions import InternalServerError
-from werkzeug.local import ContextVar
 from werkzeug.routing import BuildError
 from werkzeug.routing import Map
 from werkzeug.routing import MapAdapter
@@ -1621,13 +1620,6 @@ class Flask(Scaffold):
                 "Install Flask with the 'async' extra in order to use async views."
             ) from None
 
-        # Check that Werkzeug isn't using its fallback ContextVar class.
-        if ContextVar.__module__ == "werkzeug.local":
-            raise RuntimeError(
-                "Async cannot be used with this combination of Python "
-                "and Greenlet versions."
-            )
-
         return asgiref_async_to_sync(func)
 
     def make_response(self, rv: ResponseReturnValue) -> Response: