chore(deps): update dependency google-auth to v1.19.2 [(#4321)](GoogleCloudPlatform/python-docs-samples#4321)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [google-auth](https://togithub.com/googleapis/google-auth-library-python) | patch | `==1.19.1` -> `==1.19.2` |

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### Release Notes

<details>
<summary>googleapis/google-auth-library-python</summary>

### [`v1.19.2`](https://togithub.com/googleapis/google-auth-library-python/blob/master/CHANGELOG.md#&#8203;1192-httpswwwgithubcomgoogleapisgoogle-auth-library-pythoncomparev1191v1192-2020-07-17)

[Compare Source](https://togithub.com/googleapis/google-auth-library-python/compare/v1.19.1...v1.19.2)

</details>

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samples/to_dataframe/requirements.txt

@@ -1,4 +1,4 @@
-google-auth==1.19.1
+google-auth==1.19.2
 google-cloud-bigquery-storage==1.0.0
 google-cloud-bigquery==1.25.0
 pyarrow==0.17.1