build(deps): update flask requirement from <4,>=2.0.3 to >=3.1.3,<4

Updates the requirements on [flask](https://github.com/pallets/flask) to permit the latest version.
- [Release notes](https://github.com/pallets/flask/releases)
- [Changelog](https://github.com/pallets/flask/blob/main/CHANGES.rst)
- [Commits](pallets/flask@2.0.3...3.1.3)

---
updated-dependencies:
- dependency-name: flask
  dependency-version: 3.1.3
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements/testing.txt

@@ -1,6 +1,6 @@
 # pip install -r requirements/testing.txt
 pytest>=8.4.2,<9
 pytest-cov>=3,<8
-Flask>=2.0.3,<4 
+Flask>=3.1.3,<4 
 gevent>=22.10.2,<27
 gevent-websocket>=0.10.1,<1