Use RF 4.0.1b1 in CI

Author: aaltat

.github/workflows/CI.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       matrix:
         python-version: [3.6, 3.9, pypy3]
-        rf-version: [3.2.2, 4.0.0]
+        rf-version: [3.2.2, 4.0.1b1]
 
     steps:
     - uses: actions/checkout@v2