pytype is failing for standard `pkgutil.extend_path` usage

[tseaver]

From this Kokoro failure (there are lots):

nox > Running session pytype
nox > Creating virtual environment (virtualenv) using python3.6 in .nox/pytype
nox > python -m pip install . grpcio >= 1.8.2 grpcio-gcp >= 0.2.2 pytype >= 2019.3.21
nox > pytype
Computing dependencies
Analyzing 37 sources with 0 local dependencies
ninja: Entering directory `.pytype'
[1/37] check google.api_core.gapic_v1.routing_header
[2/37] check google.api_core.datetime_helpers
[3/37] check google.api_core.exceptions
[4/37] check google.api_core.general_helpers
[5/37] check google.api_core.timeout
[6/37] check google.api_core.retry
[7/37] check google.api_core.retry_async
[8/37] check google.api_core.version
[9/37] check google.api_core.gapic_v1.config
[10/37] check google.api_core.gapic_v1.config_async
[11/37] check google.api_core.grpc_helpers
[12/37] check google.api_core.grpc_helpers_async
[13/37] check google.api_core.client_info
[14/37] check google.api_core.gapic_v1.client_info
[15/37] check google.api_core.gapic_v1.method
[16/37] check google.api_core.gapic_v1.method_async
[17/37] check google.api_core.gapic_v1.__init__
[18/37] check google.api_core.protobuf_helpers
[19/37] check google.api_core.operations_v1.operations_client_config
[20/37] check google.api_core.page_iterator
[21/37] check google.api_core.operations_v1.operations_client
[22/37] check google.api_core.page_iterator_async
[23/37] check google.api_core.operations_v1.operations_async_client
[24/37] check google.api_core.operations_v1.__init__
[25/37] check google.api_core.future.base
[26/37] check google.api_core.future.async_future
[27/37] check google.api_core.operation_async
[28/37] check google.api_core.future._helpers
[29/37] check google.api_core.iam
[30/37] check google.api_core.future.polling
[31/37] check google.api_core.operation
[32/37] check google.__init__
FAILED: /tmpfs/src/github/python-api-core/.pytype/pyi/google/__init__.pyi
/tmpfs/src/github/python-api-core/.nox/pytype/bin/python -m pytype.single --disable pyi-error --imports_info /tmpfs/src/github/python-api-core/.pytype/imports/google.__init__.imports --module-name google.__init__ -V 3.6 -o /tmpfs/src/github/python-api-core/.pytype/pyi/google/__init__.pyi --analyze-annotated --nofail --quick /tmpfs/src/github/python-api-core/google/__init__.py
File "/tmpfs/src/github/python-api-core/google/__init__.py", line 24, in <module>: Function pkgutil.extend_path was called with the wrong arguments [wrong-arg-types]
         Expected: (path: List[str], ...)
  Actually passed: (path: Iterable[str], ...)

For more details, see https://google.github.io/pytype/errors.html#wrong-arg-types
ninja: build stopped: subcommand failed.
Leaving directory '.pytype'
nox > Command pytype  failed with exit code 1
nox > Session pytype failed.

I suspect we should upgrade to a newer version of pytype -- the current release is more than two years newer than the one we are using, and e.g. supports Python 3.9.

[tseaver]

I was able to reproduce the error locally after removing the .pytype/ cache, when running with pytype 2021.05.11. and 2021.05.06. Earlier versions (2021.04.26 and 2021.05.04) did not fail. Reported as google/pytype#918

@busunkim96, @crwilcox Should we pin pytype <= 2021.05.04 to work around this issue?

[tseaver]

I cannot reproduce this on current master (2768346) against the current release of pytype (2021.5.25).