Raise a runtime error if run_async is called without real ContextVars

Werkzeug offers a ContextVar replacement for Python < 3.7, however it
doesn't work across asyncio tasks, hence it makes sense to error out
rather than find there are odd bugs.

Note the docs build requires the latest (dev) Werkzeug due to this
change (to import ContextVar from werkzeug.local).

src/flask/helpers.py

@@ -7,6 +7,7 @@ from threading import RLock
 
 import werkzeug.utils
 from werkzeug.exceptions import NotFound
+from werkzeug.local import ContextVar
 from werkzeug.routing import BuildError
 from werkzeug.urls import url_quote
 
@@ -741,6 +742,11 @@ def run_async(func):
             "Install Flask with the 'async' extra in order to use async views."
         )
 
+    if ContextVar.__module__ == "werkzeug.local":
+        raise RuntimeError(
+            "async cannot be used with this combination of Python & Greenlet versions"
+        )
+
     @wraps(func)
     def outer(*args, **kwargs):
         """This function grabs the current context for the inner function.