chore(deps): update dependency httplib2 to v0.18.1 (GoogleCloudPlatform#3849)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [httplib2](https://togithub.com/httplib2/httplib2) | patch | `==0.18.0` -> `==0.18.1` |

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### Release Notes

<details>
<summary>httplib2/httplib2</summary>

### [`v0.18.1`](https://togithub.com/httplib2/httplib2/compare/v0.18.0...v0.18.1)

[Compare Source](https://togithub.com/httplib2/httplib2/compare/v0.18.0...v0.18.1)

</details>

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appengine/standard/mailgun/requirements.txt

@@ -1 +1 @@
-httplib2==0.18.0
+httplib2==0.18.1