chore(deps): update dependency pytest to v5.4.1 (GoogleCloudPlatform#3189)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [pytest](https://docs.pytest.org/en/latest/) | minor | `==5.3.2` -> `==5.4.1` |

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run/system-package/requirements.txt

@@ -1,3 +1,3 @@
 Flask==1.1.1
-pytest==5.3.2
+pytest==5.4.1
 gunicorn==20.0.4