Fixes pypy3 CI

requirements/test-pypy3.txt

@@ -0,0 +1 @@
+mock

tox.ini

@@ -55,6 +55,7 @@ commands = {toxinidir}/extra/release/removepyc.sh {toxinidir}
 basepython = pypy3
 deps = -r{toxinidir}/requirements/default.txt
        -r{toxinidir}/requirements/test3.txt
+       -r{toxinidir}/requirements/test-pypy3.txt
        -r{toxinidir}/requirements/test-ci.txt
        -r{toxinidir}/requirements/dev.txt
 setenv = C_DEBUG_TEST = 1