Add missing networkx dependency (#543)

The topological_code fitters module relies on networkx and has had a
hard dependency on it since it was first introduced in #211. However, it
was never added to the requirements list. This was never caught because
historically qiskit-terra (which is in the requirements list) has
required networkx too so installing qiskit-terra would install networkx.
But, in Qiskit/qiskit#5183 the dependency on networkx was removed
from terra. This commit corrects the issue so that we're properly
listing networkx as an ignis requirement moving forward. Longer term we
should migrate the topological codes fitter to use retworkx for better
performance and consistency with the rest of Qiskit. However, before we
can do that Qiskit/rustworkx#216 must be fixed first.

setup.py

@@ -21,6 +21,7 @@
 requirements = [
     "numpy>=1.13",
     "qiskit-terra>=0.13.0",
+    "networkx>=2.2",
     "scipy>=0.19,!=0.19.1",
     "setuptools>=40.1.0",
     "scikit-learn>=0.17",