*: dict.keys() returns sequence, not [] on py3

The sequence cannot be randomly accessed, e.g.

    In [5]: d = {1:2}

    In [6]: kv = d.keys()

    In [7]: kv
    Out[7]: dict_keys([1])

    In [8]: kv[0]
    ---------------------------------------------------------------------------
    TypeError                                 Traceback (most recent call last)
    <ipython-input-8-643f90e1910b> in <module>()
    ----> 1 kv[0]

    TypeError: 'dict_keys' object is not subscriptable

-> Use list(dict.keys()) in places where we need random access.

zodbtools/test/gen_testdata.py

@@ -113,7 +113,7 @@ def ext4subj(subj):
     d[xcookie] = cookie
 
     # shufle extension dict randomly - to likely trigger different ordering on save
-    keyv = d.keys()
+    keyv = list(d.keys())
     random.shuffle(keyv)
     ext = {}
     for key in keyv:
@@ -227,7 +227,7 @@ def _gen_testdb(outfs_path, zext):
                 break
 
         # delete an object
-        name = random.choice(root.keys())
+        name = random.choice(list(root.keys()))
         obj = root[name]
         root[name] = Object("%s%i*" % (name, i))
         # NOTE user/ext are kept empty on purpose - to also test this case