added missing graphviz version constraint

setup.py

@@ -31,7 +31,11 @@ setuptools.setup(
         ]
     },
     install_requires=[],
-    extras_require={"graphviz": ["graphviz"], "yaml": ["PyYAML"]},
+    extras_require={
+        # >= 0.7 subgraph context manager
+        "graphviz": ["graphviz>=0.7"],
+        "yaml": ["PyYAML"],
+    },
     setup_requires=["setuptools_scm"],
     tests_require=["pytest"],
 )