[buildbot] Add missing deepmerge dependency
Summary: D8520 is missing this new dependency which broke the buildbot.
There is no easy way to test this from the ABC repo.
Spin up a Debian container with Python installed, then:
pip install -r requirements.txt python abcbot.py
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D8522