chore(deps): update dependency google-cloud-bigquery to v2.3.1 (#371)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [google-cloud-bigquery](https://togithub.com/googleapis/python-bigquery) | minor | `==2.2.0` -> `==2.3.1` |

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### Release Notes

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

### [`v2.3.1`](https://togithub.com/googleapis/python-bigquery/blob/master/CHANGELOG.md#&#8203;231)

[Compare Source](https://togithub.com/googleapis/python-bigquery/compare/v2.2.0...v2.3.1)

11-05-2020 09:27 PST

##### Internal / Testing Changes

-   update `google.cloud.bigquery.__version__`

</details>

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samples/snippets/requirements.txt

@@ -1,4 +1,4 @@
-google-cloud-bigquery==2.2.0
+google-cloud-bigquery==2.3.1
 google-cloud-bigquery-storage==2.0.1
 google-auth-oauthlib==0.4.2
 grpcio==1.33.2