Don't install networkx with version 2.8.1 (openvinotoolkit#11718)

With new networkx release (2.8.1) some of MO tests started to fail
with following error:
```
def __setstate__(self, state):
    self._graph = G = state["_graph"]
    self._adjdict = G._pred if hasattr(G, "pred") else G._adj
    AttributeError: 'Graph' object has no attribute '_adj'
```

Seems like regression that was introduced in
networkx/networkx@f50fc70

tools/mo/requirements.txt

@@ -2,7 +2,7 @@ tensorflow>=1.15.5,<2.6
 mxnet~=1.2.0; sys_platform == 'win32'
 mxnet~=1.7.0.post2; sys_platform != 'win32'
 networkx~=2.5; python_version <= "3.6"
-networkx~=2.6; python_version > "3.6"
+networkx<2.8.1; python_version > "3.6"
 numpy>=1.16.6,<1.20
 protobuf>=3.15.6
 onnx>=1.8.1

tools/mo/requirements_caffe.txt

@@ -1,5 +1,5 @@
 networkx~=2.5; python_version <= "3.6"
-networkx~=2.6; python_version > "3.6"
+networkx<2.8.1; python_version > "3.6"
 numpy>=1.16.6,<1.20
 protobuf>=3.15.6
 defusedxml>=0.7.1

tools/mo/requirements_kaldi.txt

@@ -1,5 +1,5 @@
 networkx~=2.5; python_version <= "3.6"
-networkx~=2.6; python_version > "3.6"
+networkx<2.8.1; python_version > "3.6"
 numpy>=1.16.6,<1.20
 defusedxml>=0.7.1
 requests>=2.25.1

tools/mo/requirements_mxnet.txt

@@ -1,7 +1,7 @@
 mxnet~=1.2.0; sys_platform == 'win32'
 mxnet~=1.7.0.post2; sys_platform != 'win32'
 networkx~=2.5; python_version <= "3.6"
-networkx~=2.6; python_version > "3.6"
+networkx<2.8.1; python_version > "3.6"
 numpy>=1.16.6,<1.20
 defusedxml>=0.7.1
 urllib3>=1.26.4

tools/mo/requirements_onnx.txt

@@ -1,6 +1,6 @@
 onnx>=1.8.1
 networkx~=2.5; python_version <= "3.6"
-networkx~=2.6; python_version > "3.6"
+networkx<2.8.1; python_version > "3.6"
 numpy>=1.16.6,<1.20
 defusedxml>=0.7.1
 requests>=2.25.1

tools/mo/requirements_tf.txt

@@ -1,6 +1,6 @@
 tensorflow>=1.15.5,<2.6
 networkx~=2.5; python_version <= "3.6"
-networkx~=2.6; python_version > "3.6"
+networkx<2.8.1; python_version > "3.6"
 numpy>=1.16.6,<1.20
 defusedxml>=0.7.1
 requests>=2.25.1

tools/mo/requirements_tf2.txt

@@ -1,7 +1,7 @@
 tensorflow~=2.5.0; python_version <= "3.6"
 tensorflow~=2.5.3; python_version > "3.6"
 networkx~=2.5; python_version <= "3.6"
-networkx~=2.6; python_version > "3.6"
+networkx<2.8.1; python_version > "3.6"
 numpy>=1.16.6,<1.20
 defusedxml>=0.7.1
 requests>=2.25.1

tools/pot/setup.py

@@ -70,7 +70,7 @@ def get_version():
     "numpy>=1.16.6,<1.20",
     "addict>=2.4.0",
     "networkx~=2.5;python_version<='3.6'",
-    "networkx~=2.6;python_version>'3.6'",
+    "networkx<2.8.1;python_version>'3.6'",
     "tqdm>=4.54.1",
     "texttable~=1.6.3",
     "pandas~=1.1.5",