Update layout.py

Bring `None` comparisons in line with PEP 8: "Comparisons to singletons like None should always be done with `is` or `is not`, never the equality operators." Also, satisfy "FutureWarning: comparison to `None` will result in an elementwise object comparison in the future."

networkx/drawing/layout.py

@@ -327,7 +327,7 @@ def _fruchterman_reingold(A, dim=2, k=None, pos=None, fixed=None,
 
     A=np.asarray(A) # make sure we have an array instead of a matrix
 
-    if pos==None:
+    if pos is None:
         # random initial positions
         pos=np.asarray(np.random.random((nnodes,dim)),dtype=A.dtype)
     else:
@@ -398,15 +398,15 @@ def _sparse_fruchterman_reingold(A, dim=2, k=None, pos=None, fixed=None,
     except:
         A=(coo_matrix(A)).tolil()
 
-    if pos==None:
+    if pos is None:
         # random initial positions
         pos=np.asarray(np.random.random((nnodes,dim)),dtype=A.dtype)
     else:
         # make sure positions are of same type as matrix
         pos=pos.astype(A.dtype)
 
     # no fixed nodes
-    if fixed==None:
+    if fixed is None:
         fixed=[]
 
     # optimal distance between nodes