Add note about anyio 4.x

PKGBUILD

@@ -2,6 +2,8 @@
 
 pkgname=python-anyio
 # https://github.com/agronholm/anyio/blob/master/docs/versionhistory.rst
+# UPDATE_BLOCKED: anyio 4.x breaks at least python-fastapi
+# fastapi pins older starlette, while the latst starlette is required for anyio 4.x support https://github.com/tiangolo/fastapi/pull/9636
 pkgver=3.7.1
 pkgrel=1
 pkgdesc='High level compatibility layer for multiple asynchronous event loop implementations'