Update dependency pillow to v7.1.1 [(#3263)](GoogleCloudPlatform/python-docs-samples#3263)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [pillow](https://python-pillow.org) ([source](https://togithub.com/python-pillow/Pillow)) | patch | `==7.1.0` -> `==7.1.1` |

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### Release Notes

<details>
<summary>python-pillow/Pillow</summary>

### [`v7.1.1`](https://togithub.com/python-pillow/Pillow/blob/master/CHANGES.rst#&#8203;711-2020-04-02)

[Compare Source](https://togithub.com/python-pillow/Pillow/compare/7.1.0...7.1.1)

-   Fix regression seeking and telling PNGs [#&#8203;4512](https://togithub.com/python-pillow/Pillow/issues/4512) [#&#8203;4514](https://togithub.com/python-pillow/Pillow/issues/4514)
    [hugovk, radarhere]

</details>

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samples/snippets/crop_hints/requirements.txt

@@ -1,2 +1,2 @@
 google-cloud-vision==1.0.0
-pillow==7.1.0
+pillow==7.1.1

samples/snippets/document_text/requirements.txt

@@ -1,2 +1,2 @@
 google-cloud-vision==1.0.0
-pillow==7.1.0
+pillow==7.1.1

samples/snippets/face_detection/requirements.txt

@@ -1,2 +1,2 @@
 google-cloud-vision==1.0.0
-pillow==7.1.0
+pillow==7.1.1