flake8: disable E731: do not assign a lambda expression, use a def

It has too many false positives.  It was complaining about things like

    if this:
         fn = some_func
    else:
         fn = lambda x: ...

Where obviously, "fn" can't be a def, and it would be silly to introduce
some other name to use as the def, just to assign it to fn.

setup.cfg

@@ -14,8 +14,10 @@ ignore =
     E305,
     # E401: multiple imports on one line
     E401,
-    # too many leading '#' for block comment
+    # E266: too many leading '#' for block comment
     E266,
-    # module level import not at top of file
-    E402
+    # E402: module level import not at top of file
+    E402,
+    # E731: do not assign a lambda expression, use a def (too many false positives)
+    E731
 max-line-length = 120