Add async support

This allows for async functions to be passed to the Flask class
instance, for example as a view function,

    @app.route("/")
    async def index():
        return "Async hello"

this comes with a cost though of poorer performance than using the
sync equivalent.

asgiref is the standard way to run async code within a sync context,
and is used in Django making it a safe and sane choice for this.

CHANGES.rst

@@ -67,6 +67,8 @@ Unreleased
 -   Add route decorators for common HTTP methods. For example,
     ``@app.post("/login")`` is a shortcut for
     ``@app.route("/login", methods=["POST"])``. :pr:`3907`
+-   Support async views, error handlers, before and after request, and
+    teardown functions. :pr:`3412`
 
 
 Version 1.1.2