Add cython as a dev requirement

Cython is now needed to build, test or install from git, so it seems
reasonable to add it as a dependency.
diff --git a/requirements/dev-requirements.in b/requirements/dev-requirements.in
index 8b87107..f2e7de4 100644
--- a/requirements/dev-requirements.in
+++ b/requirements/dev-requirements.in
@@ -1,3 +1,4 @@
+cython
 pylint
 pycodestyle
 pytest >= 3.9