chore(deps): update dependency cryptography to v3.1 (#44)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [cryptography](https://togithub.com/pyca/cryptography) | minor | `==3.0` -> `==3.1` |

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### Release Notes

<details>
<summary>pyca/cryptography</summary>

### [`v3.1`](https://togithub.com/pyca/cryptography/compare/3.0...3.1)

[Compare Source](https://togithub.com/pyca/cryptography/compare/3.0...3.1)

</details>

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kms/attestations/requirements.txt

@@ -1,2 +1,2 @@
-cryptography==3.0
+cryptography==3.1
 pem==20.1.0

kms/snippets/requirements.txt

@@ -1,2 +1,2 @@
 google-cloud-kms==1.4.0
-cryptography==3.0
+cryptography==3.1