Remove stale mock dependency

Since pygraphviz no longer supports Python 2 and entirely relies
on built-in unittest.mock module, the dependency on mock is no longer
necessary.

INSTALL.txt

@@ -104,7 +104,7 @@ For Red Hat Enterprise Linux 7, the requirements are::
 
    sudo yum groups install -y "Development Tools"
    sudo yum install -y python-devel graphviz-devel
-   sudo python -m pip install nose doctest-ignore-unicode mock
+   sudo python -m pip install nose doctest-ignore-unicode
 
 
 GraphViz

requirements/test.txt

@@ -1,4 +1,3 @@
 coveralls==1.3.0
 nose==1.3.7
 doctest-ignore-unicode==0.1.2
-mock==2.0.0

setup.py

@@ -87,5 +87,5 @@
         include_package_data = True,
         test_suite='nose.collector',
         python_requires='>=3.6',
-        tests_require=['nose>=1.3.7', 'doctest-ignore-unicode>=0.1.2', 'mock>=2.0.0'],
+        tests_require=['nose>=1.3.7', 'doctest-ignore-unicode>=0.1.2'],
     )