Some trivial, almost cosmetical fixes

src/graph_tool/dl_import.py

@@ -26,7 +26,7 @@ except ImportError:
     # module
     from dl import RTLD_LAZY, RTLD_GLOBAL
 
-all = ["dl_import"]
+__all__ = ["dl_import"]
 
 
 def dl_import(import_expr):

src/graph_tool/draw/__init__.py

@@ -425,7 +425,7 @@ def graph_draw(g, pos=None, size=(15, 15), pin=False, layout=None, maxiter=None,
             aset(n, "label", "")
 
             # user supplied position
-            if pos != None:
+            if pos is not None:
                 aset(n, "pos", "%f,%f" % (pos[0][v], pos[1][v]))
                 aset(n, "pin", pin)