Further maintenance of Python dependencies

These changes probably should have been in 3b0fefb246caf9cf983d8 but
were overlooked.
diff --git a/src/python/grpcio/requirements.txt b/src/python/grpcio/requirements.txt
index 608ba40..77356e0 100644
--- a/src/python/grpcio/requirements.txt
+++ b/src/python/grpcio/requirements.txt
@@ -1,2 +1,2 @@
-enum34==1.0.4
-futures==2.2.0
+enum34>=1.0.4
+futures>=2.2.0