unpin grpcio, 1.56.0 fixed the hang?!

requirements.txt

@@ -39,9 +39,6 @@ google-cloud-ndb==2.1.1
 google-cloud-tasks==2.13.1
 googleapis-common-protos==1.59.1
 grpc-google-iam-v1==0.12.6
-# 1.55.0 hangs on Mac in gunicorn/flask run with:
-# "Waiting for thread pool to idle before forking"
-# https://github.com/grpc/grpc/issues/31885
 grpcio==1.56.0
 gunicorn==20.1.0
 html2text==2020.1.16