Update dependency google-cloud-datastore to v1.13.1 [(#4295)](GoogleCloudPlatform/python-docs-samples#4295)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [google-cloud-datastore](https://togithub.com/googleapis/python-datastore) | patch | `==1.13.0` -> `==1.13.1` |

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### Release Notes

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

### [`v1.13.1`](https://togithub.com/googleapis/python-datastore/blob/master/CHANGELOG.md#&#8203;1131-httpswwwgithubcomgoogleapispython-datastorecomparev1130v1131-2020-07-13)

[Compare Source](https://togithub.com/googleapis/python-datastore/compare/v1.13.0...v1.13.1)

</details>

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samples/snippets/requirements.txt

@@ -1,5 +1,5 @@
 google-cloud-dlp==1.0.0
 google-cloud-storage==1.29.0
 google-cloud-pubsub==1.7.0
-google-cloud-datastore==1.13.0
+google-cloud-datastore==1.13.1
 google-cloud-bigquery==1.25.0