Add virtualenv to requirements.

It doesn't seem like pytest-virtualenv depends on it, so make it
explicit.

setup.cfg

@@ -61,6 +61,7 @@ testing =
     pytest-cov
     pytest-virtualenv
     pytest-xdist
+    virtualenv
     # We keep pytest-xdist in the test dependencies, so the developer can
     # easily opt-in for distributed tests by adding, for example, the `-n 15`
     # arguments in the command-line.