Use built-in unittest.mock and remove unneeded case dep

Use the built-in unittest.mock module to implement mocks since Python 2
is no longer supported.  Remove unused fixtures, and effectively
the dependency on case package.

It seems that this package has been pulling six implicitly for
sphinx_celery, so add the transitional dep for the time being.
See celery/sphinx_celery#11.

requirements/docs.txt

@@ -1 +1,2 @@
 sphinx_celery>=1.1
+six  # missing dependency in sphinx_celery

requirements/test.txt

@@ -1,2 +1 @@
-case>=1.3.1
 pytest>=3.0

t/unit/test_funtools.py

@@ -1,6 +1,6 @@
 import pytest
 
-from case import Mock
+from unittest.mock import Mock
 
 from vine.abstract import Thenable
 from vine.funtools import (

t/unit/test_promises.py

@@ -4,10 +4,9 @@
 
 from collections import deque
 from struct import pack, unpack
+from unittest.mock import Mock
 import weakref
 
-from case import Mock
-
 from vine.funtools import wrap
 from vine.promises import promise
 

t/unit/test_synchronization.py

@@ -1,6 +1,6 @@
 import pytest
 
-from case import Mock
+from unittest.mock import Mock
 
 from vine.promises import promise
 from vine.synchronization import barrier