Stop to use the __future__ module.

The __future__ module [1] was used in this context to ensure compatibility
between python 2 and python 3.

We previously dropped the support of python 2.7 [2] and now we only support
python 3 so we don't need to continue to use this module and the imports
listed below.

Imports commonly used and their related PEPs:
- `division` is related to PEP 238 [3]
- `print_function` is related to PEP 3105 [4]
- `unicode_literals` is related to PEP 3112 [5]
- `with_statement` is related to PEP 343 [6]
- `absolute_import` is related to PEP 328 [7]

[1] https://docs.python.org/3/library/__future__.html
[2] https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html
[3] https://www.python.org/dev/peps/pep-0238
[4] https://www.python.org/dev/peps/pep-3105
[5] https://www.python.org/dev/peps/pep-3112
[6] https://www.python.org/dev/peps/pep-0343
[7] https://www.python.org/dev/peps/pep-0328

Change-Id: I5f8107287877dc5dc4a12c1f85e8671c8d49f56f

doc/source/user/tutorial/loading.rst

@@ -41,7 +41,7 @@ called directly.
 
 .. literalinclude:: ../../../../stevedore/example/load_as_driver.py
    :language: python
-   :lines: 36-37
+   :lines: 34-35
 
 Running the example program produces this output:
 
@@ -95,7 +95,7 @@ printed and then the formatted data.
 
 .. literalinclude:: ../../../../stevedore/example/load_as_extension.py
    :language: python
-   :lines: 35-39
+   :lines: 33-37
 
 The order the plugins are loaded is undefined, and depends on the
 order packages are found on the import path as well as the way the

stevedore/example/load_as_driver.py

@@ -1,5 +1,3 @@
-from __future__ import print_function
-
 import argparse
 
 from stevedore import driver

stevedore/example/load_as_extension.py

@@ -1,5 +1,3 @@
-from __future__ import print_function
-
 import argparse
 
 from stevedore import extension

stevedore/tests/test_sphinxext.py

@@ -12,8 +12,6 @@
 """Tests for the sphinx extension
 """
 
-from __future__ import unicode_literals
-
 from stevedore import extension
 from stevedore import sphinxext
 from stevedore.tests import utils