Update images to use python 3.8

After dropping support of Ansible 2.7 which has compatibility issues
with python 3.8 we now can finally upgrade to Python 3.8 which has
improvements regarding performance and memory usage.

Change-Id: I346c2fe22f8409c600a3ee2a229369778e3cabd6

.zuul.yaml

@@ -150,8 +150,8 @@
     allowed-projects: zuul/zuul
     timeout: 2700  # 45 minutes
     requires:
-      - python-builder-3.7-container-image
-      - python-base-3.7-container-image
+      - python-builder-3.8-container-image
+      - python-base-3.8-container-image
     provides: zuul-container-image
     vars: &zuul_image_vars
       docker_images:
@@ -189,8 +189,8 @@
     description: Build Docker images and upload to Docker Hub.
     allowed-projects: zuul/zuul
     requires:
-      - python-builder-3.7-container-image
-      - python-base-3.7-container-image
+      - python-builder-3.8-container-image
+      - python-base-3.8-container-image
     provides: zuul-container-image
     secrets:
       name: docker_credentials

Dockerfile

@@ -13,7 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM docker.io/opendevorg/python-builder:3.7 as builder
+FROM docker.io/opendevorg/python-builder:3.8 as builder
 ENV DEBIAN_FRONTEND=noninteractive
 
 # Optional location of Zuul API endpoint.
@@ -47,7 +47,7 @@ RUN mkdir /tmp/openshift-install \
   && echo $OPENSHIFT_SHA /tmp/openshift-install/openshift-client.tgz | sha256sum --check \
   && tar xvfz openshift-client.tgz --strip-components=1 -C /tmp/openshift-install
 
-FROM docker.io/opendevorg/python-base:3.7 as zuul
+FROM docker.io/opendevorg/python-base:3.8 as zuul
 ENV DEBIAN_FRONTEND=noninteractive
 
 COPY --from=builder /output/ /output