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.

requirements/tests.txt

@@ -4,6 +4,8 @@
 #
 #    pip-compile requirements/tests.in
 #
+asgiref==3.2.10
+    # via -r requirements/tests.in
 attrs==20.3.0
     # via pytest
 blinker==1.4