chore(deps): update precommit hook pycqa/flake8 to v3.9.0

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This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [pycqa/flake8](https://gitlab.com/pycqa/flake8) | repository | minor | `3.8.4` -> `3.9.0` |

---

### Release Notes

<details>
<summary>pycqa/flake8</summary>

### [`v3.9.0`](https://gitlab.com/pycqa/flake8/compare/3.8.4...3.9.0)

[Compare Source](https://gitlab.com/pycqa/flake8/compare/3.8.4...3.9.0)

</details>

---

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---

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---

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Source-Author: WhiteSource Renovate <[email protected]>
Source-Date: Tue Mar 23 17:38:03 2021 +0100
Source-Repo: googleapis/synthtool
Source-Sha: f5c5904fb0c6aa3b3730eadf4e5a4485afc65726
Source-Link: googleapis/synthtool@f5c5904

.pre-commit-config.yaml

@@ -12,6 +12,6 @@ repos:
     hooks:
     - id: black
 -   repo: https://gitlab.com/pycqa/flake8
-    rev: 3.8.4
+    rev: 3.9.0
     hooks:
     - id: flake8

synth.metadata

@@ -19,14 +19,14 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "79c8dd7ee768292f933012d3a69a5b4676404cda"
+        "sha": "f5c5904fb0c6aa3b3730eadf4e5a4485afc65726"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "79c8dd7ee768292f933012d3a69a5b4676404cda"
+        "sha": "f5c5904fb0c6aa3b3730eadf4e5a4485afc65726"
       }
     }
   ],