Since it's a library we might eventually start using in production
code without thinking about it, I'd rather just have it be available
in all environments.
First, all the lines of py3_dev.txt except for mypy.txt are mv-ed to
dev.txt. Then dev_lock.txt is generated from dev.txt to be used by
py3_dev.txt. `click` is removed from moto.txt since it is already a
dependency of several libraries and will appear as an autogenerated
dependency.