stateful_browser.py: fix flake8 warning (#332)

E741 ambiguous variable name 'l'

Replace this short variable name with `link`.

mechanicalsoup/stateful_browser.py

@@ -256,8 +256,8 @@ def list_links(self, *args, **kwargs):
         forwarded to :func:`links`.
         """
         print("Links in the current page:")
-        for l in self.links(*args, **kwargs):
-            print("    ", l)
+        for link in self.links(*args, **kwargs):
+            print("    ", link)
 
     def links(self, url_regex=None, link_text=None, *args, **kwargs):
         """Return links in the page, as a list of bs4.element.Tag objects.