Update mxnet version (#4081)

model-optimizer/requirements.txt

@@ -1,6 +1,6 @@
 tensorflow>=1.15.2,<2.0; python_version < "3.8"
 tensorflow>=2.2; python_version >= "3.8"
-mxnet>=1.0.0,<=1.5.1
+mxnet>=1.0.0,<=1.7.0
 networkx>=1.11
 numpy>=1.14.0,<1.19.0
 protobuf>=3.6.1

model-optimizer/requirements_mxnet.txt

@@ -1,4 +1,4 @@
-mxnet>=1.0.0,<=1.5.1
+mxnet>=1.0.0,<=1.7.0
 networkx>=1.11
 numpy>=1.14.0
 test-generator==0.1.1