[buildbot] Add missing deepmerge dependency

Summary: D8520 is missing this new dependency which broke the buildbot.

Test Plan:
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