docs(billingbudgets): add Python 2 sunset banner (via synth)

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Log from Synthtool

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/billingbudgets/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:545c758c76c3f779037aa259023ec3d1ef2d57d2c8cd00a222cb187d63ceac5e
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/cloud/billing/budgets/artman_billingbudgets_v1beta1.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/billingbudgets-v1beta1.
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/cloud/billing/budgets/v1beta1/budget_service.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/billingbudgets-v1beta1/google/cloud/billingbudgets_v1beta1/proto/budget_service.proto
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/cloud/billing/budgets/v1beta1/budget_model.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/billingbudgets-v1beta1/google/cloud/billingbudgets_v1beta1/proto/budget_model.proto
synthtool > Placed proto files into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/billingbudgets-v1beta1/google/cloud/billingbudgets_v1beta1/proto.
synthtool > Replaced 'from google\\.cloud\\.billing\\.budgets_v1beta1' in google/cloud/billing_budgets_v1beta1/proto/budget_service_pb2_grpc.py.
synthtool > Replaced 'from google\\.cloud\\.billing\\.budgets_v1beta1' in google/cloud/billing_budgets_v1beta1/proto/budget_service_pb2.py.
synthtool > Replaced 'google-cloud-billingbudgets' in google/cloud/billing_budgets_v1beta1/gapic/budget_service_client.py.
synthtool > Replaced 'google-cloud-billingbudgets' in docs/conf.py.
.coveragerc
.flake8
MANIFEST.in
docs/_static/custom.css
docs/_templates/layout.html
noxfile.py.j2
setup.cfg
Running session blacken
Creating virtual environment (virtualenv) using python3.6 in .nox/blacken
pip install black==19.3b0
black docs google tests noxfile.py setup.py
reformatted /tmpfs/src/git/autosynth/working_repo/billingbudgets/google/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/billingbudgets/google/cloud/billing_budgets.py
reformatted /tmpfs/src/git/autosynth/working_repo/billingbudgets/google/cloud/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/billingbudgets/google/cloud/billing_budgets_v1beta1/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/billingbudgets/google/cloud/billing_budgets_v1beta1/gapic/enums.py
reformatted /tmpfs/src/git/autosynth/working_repo/billingbudgets/google/cloud/billing_budgets_v1beta1/gapic/transports/budget_service_grpc_transport.py
reformatted /tmpfs/src/git/autosynth/working_repo/billingbudgets/google/cloud/billing_budgets_v1beta1/gapic/budget_service_client_config.py
reformatted /tmpfs/src/git/autosynth/working_repo/billingbudgets/google/cloud/billing_budgets_v1beta1/proto/budget_model_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/billingbudgets/docs/conf.py
reformatted /tmpfs/src/git/autosynth/working_repo/billingbudgets/google/cloud/billing_budgets_v1beta1/gapic/budget_service_client.py
reformatted /tmpfs/src/git/autosynth/working_repo/billingbudgets/google/cloud/billing_budgets_v1beta1/proto/budget_service_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/billingbudgets/google/cloud/billing_budgets_v1beta1/types.py
reformatted /tmpfs/src/git/autosynth/working_repo/billingbudgets/noxfile.py
reformatted /tmpfs/src/git/autosynth/working_repo/billingbudgets/tests/unit/gapic/v1beta1/test_budget_service_client_v1beta1.py
reformatted /tmpfs/src/git/autosynth/working_repo/billingbudgets/google/cloud/billing_budgets_v1beta1/proto/budget_service_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/billingbudgets/google/cloud/billing_budgets_v1beta1/proto/budget_model_pb2.py
All done! ✨ 🍰 ✨
16 files reformatted, 4 files left unchanged.
Session blacken was successful.
synthtool > Cleaned up 2 temporary directories.
synthtool > Wrote metadata to synth.metadata.