Update dependency flask to v3.1.2

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
flask (changelog)	==3.1.1 -> ==3.1.2

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Release Notes

pallets/flask (flask)

v3.1.2

Compare Source

Released 2025-08-19

• stream_with_context does not fail inside async views. :issue:5774
• When using follow_redirects in the test client, the final state
  of session is correct. :issue:5786
• Relax type hint for passing bytes IO to send_file. :issue:5776

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