tests: introduce doctest_mock_ugettext to allow doctests of localized code

Future doctests will require some extra mocking, as the code-under-test uses
translation (ugettext aka '_') and its provider TurboGears2 needs a context.

Avoid this complexity by mocking ugettext as the identity function. This is
done by providing a pytest fixture 'doctest_mock_ugettext' that will mock
uggettext in the module that uses the fixture.

conftest.py

@@ -22,3 +22,16 b' def pytest_ignore_collect(path):'
     )
     if str(path).endswith(kallithea_ignore_paths):
         return True
+
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+def doctest_mock_ugettext(request):
+    """Mock ugettext ('_') in the module using this fixture.
+
+    Intended to be used for doctests.
+
+    In a doctest, enable this fixture using:
+        >>> getfixture('doctest_mock_ugettext')
+    """
+    m = __import__(request.module.__name__, globals(), locals(), [None], 0)
+    with mock.patch.object(m, '_', lambda s: s):
+        yield