Update Cirrus config to use the legacy resolver of pip

Author: sdispater

.cirrus.yml

@@ -16,7 +16,7 @@ test_task:
   pip_script:
     - $PYTHON -m ensurepip
     - $PYTHON -m pip install -U pip tox
-    - $PYTHON -m pip install -U --pre poetry
+    - $PYTHON -m pip install --use-deprecated=legacy-resolver -U --pre poetry
     - poetry config virtualenvs.in-project true
   tox_script: $PYTHON -m tox -e py -- -q --junitxml=junit.xml tests
   on_failure: