chore(deps): update dependency google-cloud-pubsub to v2.6.1 (#175)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [google-cloud-pubsub](https://togithub.com/googleapis/python-pubsub) | `==2.6.0` -> `==2.6.1` | [![age](https://badges.renovateapi.com/packages/pypi/google-cloud-pubsub/2.6.1/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/google-cloud-pubsub/2.6.1/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/google-cloud-pubsub/2.6.1/compatibility-slim/2.6.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/google-cloud-pubsub/2.6.1/confidence-slim/2.6.0)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

<details>
<summary>googleapis/python-pubsub</summary>

### [`v2.6.1`](https://togithub.com/googleapis/python-pubsub/blob/master/CHANGELOG.md#&#8203;261)

[Compare Source](https://togithub.com/googleapis/python-pubsub/compare/v2.6.0...v2.6.1)

07-05-2021 10:33 PDT

##### Dependencies

-   Fix possible crash by requiring `grpcio >= 1.38.1`. ([#&#8203;414](https://togithub.com/googleapis/python-pubsub/issues/414)) ([7037a28](https://togithub.com/googleapis/python-pubsub/pull/435/commits/7037a28090aa4efa01808231721716bca80bb0b7))

##### Documentation

-   Adjust samples for publishing with error handler and flow control. ([#&#8203;433](https://togithub.com/googleapis/python-pubsub/pull/433))

##### Internal / Testing Changes

-   Fix flaky sync pull sample test. ([#&#8203;434](https://togithub.com/googleapis/python-pubsub/pull/434))
-   Mitigate flaky snippets tests. ([#&#8203;432](https://togithub.com/googleapis/python-pubsub/pull/432))

</details>

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packages/google-cloud-bigquery-datatransfer/samples/snippets/requirements-test.txt

@@ -1,4 +1,4 @@
 google-cloud-bigquery==2.20.0
-google-cloud-pubsub==2.6.0
+google-cloud-pubsub==2.6.1
 pytest==6.2.4
 mock==4.0.3