chore(deps): update dependency pillow to v8.1.2 (googleapis#115)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [pillow](https://python-pillow.org) ([source](https://togithub.com/python-pillow/Pillow), [changelog](https://togithub.com/python-pillow/Pillow/blob/master/CHANGES.rst)) | `==8.1.1` -> `==8.1.2` | [![age](https://badges.renovateapi.com/packages/pypi/pillow/8.1.2/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/pillow/8.1.2/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/pillow/8.1.2/compatibility-slim/8.1.1)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/pillow/8.1.2/confidence-slim/8.1.1)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v8.1.2`](https://togithub.com/python-pillow/Pillow/blob/master/CHANGES.rst#&#8203;812-2021-03-06)

[Compare Source](https://togithub.com/python-pillow/Pillow/compare/8.1.1...8.1.2)

-   Fix Memory DOS in BLP (CVE-2021-27921), ICNS (CVE-2021-27922) and ICO (CVE-2021-27923) Image Plugins
    [wiredfool]

</details>

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samples/snippets/crop_hints/requirements.txt

@@ -1,2 +1,2 @@
 google-cloud-vision==2.2.0
-pillow==8.1.1
+pillow==8.1.2

samples/snippets/document_text/requirements.txt

@@ -1,2 +1,2 @@
 google-cloud-vision==2.2.0
-pillow==8.1.1
+pillow==8.1.2

samples/snippets/face_detection/requirements.txt

@@ -1,2 +1,2 @@
 google-cloud-vision==2.2.0
-pillow==8.1.1
+pillow==8.1.2