Pin back version of more-itertools so tox works

More context: https://github.com/tox-dev/tox/issues/1414

Change-Id: Ide063f70ad5f58c8d45a990de2cc3ed9f6b09795
diff --git a/packer/provision/basebuild.sh b/packer/provision/basebuild.sh
index 1bba231..23fb474 100644
--- a/packer/provision/basebuild.sh
+++ b/packer/provision/basebuild.sh
@@ -135,6 +135,7 @@
         grpcio-tools \
         isort \
         linkchecker \
+        more-itertools==5.0.0 \
         mock \
         nose2 \
         pexpect \