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).

tox.ini

@@ -25,5 +25,8 @@ skip_install = true
 commands = pre-commit run --all-files --show-diff-on-failure
 
 [testenv:docs]
-deps = -r requirements/docs.txt
+deps =
+    -r requirements/docs.txt
+
+    https://github.com/pallets/werkzeug/archive/master.tar.gz
 commands = sphinx-build -W -b html -d {envtmpdir}/doctrees docs {envtmpdir}/html