Add mock to dev reequirements now that we use it

I used it in e46c4eb, but forgot to add it to requirements since the
virtualenv I was using already had it.

Test plan: `make deps check`

Auditors: csilvers

Author: benjaminjkraft

requirements.dev.txt

@@ -1,2 +1,3 @@
+mock
 flake8
 twine