MAINT: Make walk usage in example tests compatible with Python 3

The rename of walk is not provided by six, so check the import error
via an exception.

Also, callback behavior slightly changes between the two versions,
so instead iterate over the walked files and call what was formerly
a callback, directly as a function.

tests/test_examples.py

@@ -21,15 +21,22 @@
 import matplotlib
 matplotlib.use('Agg')
 
-from os import path
 import os
+from os import path
+
+try:
+    from path import walk
+except ImportError:
+    # Assume Python 3
+    from os import walk
+
 import fnmatch
 import imp
 
 
 def test_examples():
     os.chdir(example_dir())
-    for fname in all_matching_files('.', '*.py'):
+    for fname in all_matching_files(example_dir(), '*.py'):
         yield check_example, fname
 
 
@@ -40,7 +47,8 @@ def all_matching_files(d, pattern):
         fls.extend(nfiles)
 
     files = []
-    path.walk(d, addfiles, files)
+    for dirpath, dirnames, filenames in walk(d):
+        addfiles(files, dirpath, filenames)
     return files