pinning `msgpack` to >=1.0.0,<2.0.0

syncing with rllib msgpack version requirement (dependency resolution was getting failed)

requirements_dev.txt

@@ -8,7 +8,7 @@ numpy>=1.16.2
 recordtype>=1.3
 matplotlib>=3.0.2
 Pillow>=5.4.1
-msgpack==0.6.1
+msgpack>=1.0.0,<2.0.0
 msgpack-numpy>=0.4.4.0
 svgutils>=0.3.1
 pyarrow>=0.13.0