setup: Install mock for Python 3 too

'mock' is part of the standard library in Python 3 since Python 3.3.
However, it's found in 'unittest.mock' - not 'mock'. We should install
the version for PyPi in all cases to minimize differences between the
two. When Python 2.7 support is dropped, we can consider switching to
the standard library version.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Fixes #4284

setup.py

@@ -68,13 +68,13 @@ extras_require = {
         'whoosh>=2.0',
     ],
     'test': [
+        'mock',
         'pytest',
         'pytest-cov',
         'html5lib',
     ],
     'test:python_version<"3"': [
         'enum34',
-        'mock',
     ],
     'test:python_version>="3"': [
         'mypy',