build(deps): update flask requirement from <4,>=2.0.3 to >=3.1.3,<4 (#107)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.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>=7.1.0,<8
-Flask>=2.0.3,<4 
+Flask>=3.1.3,<4 
 gevent>=22.10.2,<27
 gevent-websocket>=0.10.1,<1