chore(deps): update dependency google-cloud-securitycenter to v0.7.1 (#58)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [google-cloud-securitycenter](https://togithub.com/googleapis/python-securitycenter) | minor | `==0.6.0` -> `==0.7.1` |

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### Release Notes

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

### [`v0.7.1`](https://togithub.com/googleapis/python-securitycenter/blob/master/CHANGELOG.md#&#8203;071-httpswwwgithubcomgoogleapispython-securitycentercomparev070v071-2020-09-18)

[Compare Source](https://togithub.com/googleapis/python-securitycenter/compare/v0.7.0...v0.7.1)

### [`v0.7.0`](https://togithub.com/googleapis/python-securitycenter/blob/master/CHANGELOG.md#&#8203;070-httpswwwgithubcomgoogleapispython-securitycentercomparev060v070-2020-09-10)

[Compare Source](https://togithub.com/googleapis/python-securitycenter/compare/v0.6.0...v0.7.0)

##### Features

-   add field severity to findings; update retry configs ([#&#8203;53](https://www.github.com/googleapis/python-securitycenter/issues/53)) ([80494a9](https://www.github.com/googleapis/python-securitycenter/commit/80494a915ca33d260862694be889b817869ff01a))

##### Documentation

-   Update Security Command Center UpdateNotificationConfig sample, adding filter to mutable field ([#&#8203;39](https://www.github.com/googleapis/python-securitycenter/issues/39)) ([c70d790](https://www.github.com/googleapis/python-securitycenter/commit/c70d7904425ae5ac252ffa7317ec6d08234a6c27))

</details>

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securitycenter/snippets/requirements.txt

@@ -1,2 +1,2 @@
 google-cloud-pubsub==1.7.0
-google-cloud-securitycenter==0.6.0
+google-cloud-securitycenter==0.7.1