MNT: Remove six/backported mock

requirements.txt

@@ -2,5 +2,4 @@ h5py>=2.9
 nibabel>=2.4
 numpy>=1
 scipy>=0.18
-six>=1
 dataclasses

setup.py

@@ -8,8 +8,9 @@
 
 from __future__ import print_function
 
-import os.path as op
-import            shutil
+import os.path       as op
+import                  shutil
+import unittest.mock as mock
 
 from setuptools import setup
 from setuptools import find_namespace_packages
@@ -68,11 +69,6 @@ class doc(Command):
 
         import sphinx.cmd.build as sphinx_build
 
-        try:
-            import unittest.mock as mock
-        except:
-            import mock
-
         mockobj       = mock.MagicMock()
         mockobj.__version__ = '2.2.0'
         mockedModules = open(op.join(docdir, 'mock_modules.txt')).readlines()