Further maintenance of Python dependencies

These changes probably should have been in 3b0fefb246 but
were overlooked.

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

src/python/grpcio/setup.py

@@ -87,9 +87,8 @@ _PACKAGE_DIRECTORIES = {
 }
 
 _INSTALL_REQUIRES = (
-    'enum34==1.0.4',
-    'futures==2.2.0',
-    'protobuf==3.0.0a3',
+    'enum34>=1.0.4',
+    'futures>=2.2.0',
 )
 
 _SETUP_REQUIRES = (

src/python/grpcio_test/setup.py

@@ -72,6 +72,8 @@ _SETUP_REQUIRES = (
 _INSTALL_REQUIRES = (
     'oauth2client>=1.4.7',
     'grpcio>=0.11.0b0',
+    # TODO(issue 3321): Unpin protobuf dependency.
+    'protobuf==3.0.0a3',
 )
 
 _COMMAND_CLASS = {