chore(python): remove obsolete release scripts and config files

Source-Link: googleapis/synthtool@6357517
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:91d0075c6f2fd6a073a06168feee19fa2a8507692f2519a1dc7de3366d157e99

Author: gcf-owl-bot[bot]

.github/.OwlBot.lock.yaml

@@ -13,5 +13,5 @@
 # limitations under the License.
 docker:
   image: gcr.io/cloud-devrel-public-resources/owlbot-python:latest
-  digest: sha256:5efdf8d38e5a22c1ec9e5541cbdfde56399bdffcb6f531183f84ac66052a8024
-# created: 2024-10-25
+  digest: sha256:91d0075c6f2fd6a073a06168feee19fa2a8507692f2519a1dc7de3366d157e99
+# created: 2024-11-11T16:13:09.302418532Z

.kokoro/test-samples-impl.sh

@@ -33,7 +33,8 @@ export PYTHONUNBUFFERED=1
 env | grep KOKORO
 
 # Install nox
-python3.9 -m pip install --upgrade --quiet nox
+# `virtualenv==20.26.6` is added for Python 3.7 compatibility
+python3.9 -m pip install --upgrade --quiet nox virtualenv==20.26.6
 
 # Use secrets acessor service account to get secrets
 if [[ -f "${KOKORO_GFILE_DIR}/secrets_viewer_service_account.json" ]]; then