pin networkx to 2.5 (#6085)

I am bit hesitant to add an upper-bound here
as networkx seems to be requiring a lot of
dependencies from next version onwards.

setup.py

@@ -73,7 +73,7 @@ install_requires = [
     "flufl.lock>=3.2,<4",
     "win-unicode-console>=0.5; sys_platform == 'win32'",
     "pywin32>=225; sys_platform == 'win32'",
-    "networkx>=2.1",
+    "networkx~=2.5",
     "psutil>=5.8.0",
     "pydot>=1.2.4",
     "speedcopy>=2.0.1; python_version < '3.8' and sys_platform == 'win32'",