chore(deps): update dependency google-cloud-pubsub to v2.4.0 (#110)

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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) | `==2.3.0` -> `==2.4.0` | [![age](https://badges.renovateapi.com/packages/pypi/google-cloud-pubsub/2.4.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/google-cloud-pubsub/2.4.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/google-cloud-pubsub/2.4.0/compatibility-slim/2.3.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/google-cloud-pubsub/2.4.0/confidence-slim/2.3.0)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v2.4.0`](https://togithub.com/googleapis/python-pubsub/blob/master/CHANGELOG.md#&#8203;240)

[Compare Source](https://togithub.com/googleapis/python-pubsub/compare/v2.3.0...v2.4.0)

02-22-2021 05:02 PST

##### Implementation Changes

##### New Features

-   Add graceful streaming pull shutdown. ([#&#8203;292](https://togithub.com/googleapis/python-pubsub/pull/292))

##### Documentation

-   Update samples with using the subscriber client as a context manager. ([#&#8203;254](https://togithub.com/googleapis/python-pubsub/pull/254))

</details>

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dlp/snippets/requirements.txt

@@ -1,5 +1,5 @@
 google-cloud-dlp==3.0.1
 google-cloud-storage==1.36.1
-google-cloud-pubsub==2.3.0
+google-cloud-pubsub==2.4.0
 google-cloud-datastore==2.1.0
 google-cloud-bigquery==2.9.0