chore(deps): update dependency redis to v3.5.0 (GoogleCloudPlatform#3583)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [redis](https://togithub.com/andymccurdy/redis-py) | minor | `==3.4.1` -> `==3.5.0` |

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### Release Notes

<details>
<summary>andymccurdy/redis-py</summary>

### [`v3.5.0`](https://togithub.com/andymccurdy/redis-py/compare/3.4.1...3.5.0)

[Compare Source](https://togithub.com/andymccurdy/redis-py/compare/3.4.1...3.5.0)

</details>

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appengine/flexible/redis/requirements.txt

@@ -1,3 +1,3 @@
 Flask==1.1.2
 gunicorn==20.0.4
-redis==3.4.1
+redis==3.5.0

appengine/standard_python37/redis/requirements.txt

@@ -1,2 +1,2 @@
 Flask==1.1.2
-redis==3.4.1
+redis==3.5.0

functions/memorystore/redis/requirements.txt

@@ -1 +1 @@
-redis==3.4.1
+redis==3.5.0

memorystore/redis/requirements.txt

@@ -13,5 +13,5 @@
 # [START memorystore_requirements]
 Flask==1.1.2
 gunicorn==20.0.4
-redis==3.4.1
+redis==3.5.0
 # [END memorystore_requirements]