Fix version of cryptography to v3.1.1 for Travis CI PyPy3.5 runner (#322)

tox.ini

@@ -88,7 +88,10 @@ commands =
 deps=
    -r{toxinidir}/requirements.txt
    -r{toxinidir}/dev-requirements.txt
+# Last supported cryptography version that can link against
+# OpenSSL v1.0.2 (which pypy35 uses) is 3.1.1
 commands =
+    pip install --upgrade cryptography==3.1.1
     python setup.py compile_catalog
     coverage run --parallel -m pytest {posargs}
     flake8 . --count --show-source --statistics