chore(deps): update dependency google-cloud-pubsub to v2.8.0 (#218)

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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) | `==2.7.1` -> `==2.8.0` | [![age](https://badges.renovateapi.com/packages/pypi/google-cloud-pubsub/2.8.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/google-cloud-pubsub/2.8.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/google-cloud-pubsub/2.8.0/compatibility-slim/2.7.1)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/google-cloud-pubsub/2.8.0/confidence-slim/2.7.1)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v2.8.0`](https://togithub.com/googleapis/python-pubsub/blob/master/CHANGELOG.md#&#8203;280-httpswwwgithubcomgoogleapispython-pubsubcomparev271v280-2021-09-02)

[Compare Source](https://togithub.com/googleapis/python-pubsub/compare/v2.7.1...v2.8.0)

##### Features

-   closed subscriber as context manager raises ([#&#8203;488](https://www.togithub.com/googleapis/python-pubsub/issues/488)) ([a05a3f2](https://www.github.com/googleapis/python-pubsub/commit/a05a3f250cf8567ffe0d2eb3ecc45856a2bcd07c))

##### Documentation

-   clarify the types of Message parameters ([#&#8203;486](https://www.togithub.com/googleapis/python-pubsub/issues/486)) ([633e91b](https://www.github.com/googleapis/python-pubsub/commit/633e91bbfc0a8f4f484089acff6812b754f40c75))

##### [2.7.1](https://www.github.com/googleapis/python-pubsub/compare/v2.7.0...v2.7.1) (2021-08-13)

##### Bug Fixes

-   remove dependency on pytz ([#&#8203;472](https://www.togithub.com/googleapis/python-pubsub/issues/472)) ([972cc16](https://www.github.com/googleapis/python-pubsub/commit/972cc163f5a1477b37a5ab7e329faf1468637fa2))

</details>

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bigquery-datatransfer/snippets/requirements-test.txt

@@ -1,4 +1,4 @@
 google-cloud-bigquery==2.26.0
-google-cloud-pubsub==2.7.1
+google-cloud-pubsub==2.8.0
 pytest==6.2.5
 mock==4.0.3