networkx raises TypeError when reading input from .gml file

Hello,
I'm reporting the Debian bug #735776 : 

```
networkx raises TypeError when reading input from .gml file:

$ python cart_product.py c3.gml c3.gml
graph [
  node [
    id 0
Traceback (most recent call last):
  File "cart_product.py", line 22, in <module>
    for line in nx.generate_gml(X):
  File "/usr/lib/pymodules/python2.7/networkx/readwrite/gml.py", line 328, in generate_gml
    yield 2*indent+'label %s'%label
TypeError: not all arguments converted during string formatting

[8<]

It seems that bug is fixed by applying the following patch (tested for
debian stable and sid):

--- a/networkx/readwrite/gml.py
+++ b/networkx/readwrite/gml.py
@@ -325,7 +325,7 @@
         label=G.node[n].get('label',n)
         if is_string_like(label):
             label='"%s"'%label
-        yield 2*indent+'label %s'%label
+        yield 2*indent+'label {0}'.format(label)
         if n in G:
           for k,v in G.node[n].items():
               if k=='id' or k == 'label': continue
```