Add docker-compose as a requirement of certbot-ci (#7120)

Fixes #7110 

This PR declares docker-compose as a requirement for certbot-ci. This way, a recent version of docker-compose is installed in the standard virtual environment set up by `tools/venv.py` and `tools/venv3.py`, and so is available to pytest integration tests from `tox` or in the virtual environment enabled.

* Add docker-compose as a dev dependency and declares it in certbot-ci requirements

* Update docker-compose 1.25.0

certbot-ci/certbot_integration_tests/conftest.py

@@ -6,6 +6,7 @@ for a directory a specific configuration using built-in pytest hooks.
 
 See https://docs.pytest.org/en/latest/reference.html#hook-reference
 """
+from __future__ import print_function
 import contextlib
 import subprocess
 import sys

certbot-ci/setup.py

@@ -10,6 +10,7 @@ version = '0.32.0.dev0'
 install_requires = [
     'coverage',
     'cryptography',
+    'docker-compose',
     'pyopenssl',
     'pytest',
     'pytest-cov',