Use Python 3.7 to run ansible-test coverage in CI. (#57555)

Generating XML for coverage reports on Python 3.7 uses far less
memory than on Python 2.7. This change should avoid the MemoryError
we've been seeing in the nightly coverage runs.

test/utils/shippable/shippable.sh

@@ -81,6 +81,13 @@ function cleanup
             stub=""
         fi
 
+        # use python 3.7 for coverage to avoid running out of memory during coverage xml processing
+        # only use it for coverage to avoid the additional overhead of setting up a virtual environment for a potential no-op job
+        virtualenv --python /usr/bin/python3.7 ~/ansible-venv
+        set +ux
+        . ~/ansible-venv/bin/activate
+        set -ux
+
         # shellcheck disable=SC2086
         ansible-test coverage xml --color -v --requirements --group-by command --group-by version ${stub:+"$stub"}
         cp -a test/results/reports/coverage=*.xml shippable/codecoverage/