Pin anyio version to below 4.0.0 (#916)

Author: lrafeei

tox.ini

@@ -340,6 +340,12 @@ deps =
     framework_sanic-saniclatest: sanic
     framework_sanic-sanic{1812,190301,1906}: aiohttp
     framework_sanic-sanic{1812,190301,1906,1912,200904,210300,2109,2112,2203,2290}: websockets<11
+    ; For test_exception_in_middleware test, anyio is used:
+    ; https://github.com/encode/starlette/pull/1157
+    ; but anyiolatest creates breaking changes to our tests 
+    ; (but not the instrumentation):
+    ; https://github.com/agronholm/anyio/releases/tag/4.0.0
+    framework_starlette: anyio<4
     framework_starlette-starlette0014: starlette<0.15
     framework_starlette-starlette0015: starlette<0.16
     framework_starlette-starlette0019: starlette<0.20