CI: Fix linter errors for Kokoro samples tests for Python >3.6

[acocuzzo]

Kokoro lint run fails with:

nox > flake8 --show-source --builtin=gettext --max-complexity=20 --import-order-style=google '--exclude=.nox,.cache,env,lib,generated_pb2,*_pb2.py,*_pb2_grpc.py' --ignore=E121,E123,E126,E203,E226,E24,E266,E501,E704,W503,W504,I202 --max-line-length=88 --application-import-names publisher,subscriber,noxfile_config,noxfile,schema,iam_test,resources,publisher_test,iam,quickstart,utilities,.nox,subscriber_test,schema_test .
./subscriber_test.py:144:7: ANN401 Dynamically typed expressions (typing.Any) are disallowed
    **attrs: Any,
      ^
nox > Command flake8 --show-source --builtin=gettext --max-complexity=20 --import-order-style=google '--exclude=.nox,.cache,env,lib,generated_pb2,*_pb2.py,*_pb2_grpc.py' --ignore=E121,E123,E126,E203,E226,E24,E266,E501,E704,W503,W504,I202 --max-line-length=88 --application-import-names publisher,subscriber,noxfile_config,noxfile,schema,iam_test,resources,publisher_test,iam,quickstart,utilities,.nox,subscriber_test,schema_test . failed with exit code 1
nox > Session lint failed.