chore(deps): update dependency pyyaml to v5.3.1 (GoogleCloudPlatform#3191)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [pyyaml](https://togithub.com/yaml/pyyaml) | minor | `==5.1.2` -> `==5.3.1` |
| [pyyaml](https://togithub.com/yaml/pyyaml) | minor | `==5.2` -> `==5.3.1` |

---

### Release Notes

<details>
<summary>yaml/pyyaml</summary>

### [`v5.3.1`](https://togithub.com/yaml/pyyaml/compare/5.3...5.3.1)

[Compare Source](https://togithub.com/yaml/pyyaml/compare/5.3...5.3.1)

### [`v5.3`](https://togithub.com/yaml/pyyaml/compare/5.2...5.3)

[Compare Source](https://togithub.com/yaml/pyyaml/compare/5.2...5.3)

### [`v5.2`](https://togithub.com/yaml/pyyaml/blob/master/CHANGES#&#8203;52-2019-12-02)

[Compare Source](https://togithub.com/yaml/pyyaml/compare/5.1.2...5.2)

-   Repair incompatibilities introduced with 5.1. The default Loader was changed,
    but several methods like add_constructor still used the old default
    [#&GoogleCloudPlatform#8203;279](https://togithub.com/yaml/pyyaml/pull/279) -- A more flexible fix for custom tag constructors
    [#&GoogleCloudPlatform#8203;287](https://togithub.com/yaml/pyyaml/pull/287) -- Change default loader for yaml.add_constructor
    [#&GoogleCloudPlatform#8203;305](https://togithub.com/yaml/pyyaml/pull/305) -- Change default loader for add_implicit_resolver, add_path_resolver
-   Make FullLoader safer by removing python/object/apply from the default FullLoader
    [#&GoogleCloudPlatform#8203;347](https://togithub.com/yaml/pyyaml/pull/347) -- Move constructor for object/apply to UnsafeConstructor
-   Fix bug introduced in 5.1 where quoting went wrong on systems with sys.maxunicode &lt;= 0xffff
    [#&GoogleCloudPlatform#8203;276](https://togithub.com/yaml/pyyaml/pull/276) -- Fix logic for quoting special characters
-   Other PRs:
    [#&GoogleCloudPlatform#8203;280](https://togithub.com/yaml/pyyaml/pull/280) -- Update CHANGES for 5.1

</details>

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endpoints/getting-started/requirements.txt

@@ -2,7 +2,7 @@ Flask==1.1.1
 flask-cors==3.0.8
 gunicorn==20.0.4
 six==1.13.0
-pyyaml==5.2
+pyyaml==5.3.1
 requests==2.22.0
 google-auth==1.11.2
 google-auth-oauthlib==0.4.1

testing/requirements.txt

@@ -7,7 +7,7 @@ PyCrypto==2.6.1
 pytest-cov==2.8.1
 pytest==5.3.2; python_version > "3.0"
 pytest==4.6.9; python_version < "3.0"
-pyyaml==5.1.2
+pyyaml==5.3.1
 responses==0.10.6
 WebTest==2.0.34
 webapp2==2.5.2