Update to the latest version of pytype

Owl bot PR #251 failed the pytype check with seemingly unrelated changes. It turns out that the check passes with an older version of pytype. I've [pinned pytype](https://github.com/googleapis/python-pubsublite/pull/251/commits/bda682e8c8da82c08b5e729fe1f644288f093e97) to version `2021.09.09` , the last version where the check passes.

Steps to reproduce
1. Install `pytype==2021.09.09`
2. Run `pytype google/cloud/pubsublite/internal/constructable_from_service_account.py`
```
Computing dependencies
Analyzing 1 sources with 0 local dependencies
ninja: Entering directory `.pytype'
ninja: no work to do.
Leaving directory '.pytype'
Success: no errors found
```
3. Install `pytype==2021.09.27` or a newer version of `pytype`
4. Run `pytype google/cloud/pubsublite/internal/constructable_from_service_account.py`
```
Computing dependencies
Analyzing 1 sources with 0 local dependencies
ninja: Entering directory `.pytype'
[1/1] check google.cloud.pubsublite.internal.constructable_from_service_account
FAILED: /usr/local/google/home/partheniou/git/python-pubsublite/.pytype/pyi/google/cloud/pubsublite/internal/constructable_from_service_account.pyi
/usr/local/google/home/partheniou/.pyenv/versions/3.9.2/envs/test1/bin/python -m pytype.single --imports_info /usr/local/google/home/partheniou/git/python-pubsublite/.pytype/imports/google.cloud.pubsublite.internal.constructable_from_service_account.imports --module-name google.cloud.pubsublite.internal.constructable_from_service_account -V 3.9 -o /usr/local/google/home/partheniou/git/python-pubsublite/.pytype/pyi/google/cloud/pubsublite/internal/constructable_from_service_account.pyi --analyze-annotated --nofail --quick /usr/local/google/home/partheniou/git/python-pubsublite/google/cloud/pubsublite/internal/constructable_from_service_account.py
File "/usr/local/google/home/partheniou/git/python-pubsublite/google/cloud/pubsublite/internal/constructable_from_service_account.py", line 30, in from_service_account_file: Invalid keyword argument credentials to function ConstructableFromServiceAccount.__init__ [wrong-keyword-args]
Expected: (self)
Actually passed: (self, credentials)

For more details, see https://google.github.io/pytype/errors.html#wrong-keyword-args
ninja: build stopped: subcommand failed.
Leaving directory '.pytype'
```