pass file mode to h5py.File() (#3165) (#3166)

* force different versions of h5py in CI

* pass file modes to h5py.File

ml-agents/mlagents/trainers/buffer.py

         """
         Saves the AgentBuffer to a file-like object.
         """
-        with h5py.File(file_object) as write_file:
+        with h5py.File(file_object, "w") as write_file:
             for key, data in self.items():
                 write_file.create_dataset(key, data=data, dtype="f", compression="gzip")
 
         """
-        with h5py.File(file_object) as read_file:
+        with h5py.File(file_object, "r") as read_file:
             for key in list(read_file.keys()):
                 self[key] = AgentBuffer.AgentBufferField()
                 # extend() will convert the numpy array's first dimension into list

test_constraints_max_tf1_version.txt

 grpcio>=1.23.0
 numpy>=1.17.2
 tensorflow>=1.14.0,<2.0
+h5py>=2.10.0

test_constraints_max_tf2_version.txt

 grpcio>=1.23.0
 numpy>=1.17.2
 tensorflow>=2.0.0,<2.1.0
+h5py>=2.10.0

test_constraints_min_version.txt

 Pillow==4.2.1
 protobuf==3.6
 tensorflow==1.7
+h5py==2.9.0