chore(deps): update dependency google-cloud-pubsub to v1.7.0 [(#4290)](GoogleCloudPlatform/python-docs-samples#4290)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [google-cloud-pubsub](https://togithub.com/googleapis/python-pubsub) | minor | `==1.6.1` -> `==1.7.0` |

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### Release Notes

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

### [`v1.7.0`](https://togithub.com/googleapis/python-pubsub/blob/master/CHANGELOG.md#&#8203;170-httpswwwgithubcomgoogleapispython-pubsubcomparev161v170-2020-07-13)

[Compare Source](https://togithub.com/googleapis/python-pubsub/compare/v1.6.1...v1.7.0)

##### New Features

-   Add support for server-side flow control. ([#&#8203;143](https://togithub.com/googleapis/python-pubsub/pull/143)) ([04e261c](https://www.github.com/googleapis/python-pubsub/commit/04e261c602a2919cc75b3efa3dab099fb2cf704c))

##### Dependencies

-   Update samples dependency `google-cloud-pubsub` to `v1.6.1`. ([#&#8203;144](https://togithub.com/googleapis/python-pubsub/pull/144)) ([1cb6746](https://togithub.com/googleapis/python-pubsub/commit/1cb6746b00ebb23dbf1663bae301b32c3fc65a88))

##### Documentation

-   Add pubsub/cloud-client samples from the common samples repo (with commit history). ([#&#8203;151](https://togithub.com/googleapis/python-pubsub/pull/151)) 
-   Add flow control section to publish overview. ([#&#8203;129](https://togithub.com/googleapis/python-pubsub/pull/129)) ([acc19eb](https://www.github.com/googleapis/python-pubsub/commit/acc19eb048eef067d9818ef3e310b165d9c6307e))
-   Add a link to Pub/Sub filtering language public documentation to `pubsub.proto`. ([#&#8203;121](https://togithub.com/googleapis/python-pubsub/pull/121)) ([8802d81](https://www.github.com/googleapis/python-pubsub/commit/8802d8126247f22e26057e68a42f5b5a82dcbf0d))

</details>

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samples/snippets/requirements.txt

@@ -1,4 +1,4 @@
-google-cloud-pubsub==1.6.1
+google-cloud-pubsub==1.7.0
 google-cloud-containeranalysis==0.3.1
 grafeas==0.4.1
 pytest==5.3.0; python_version > "3.0"