Update dependency Flask to v3.1.2

[magicalbob]

This PR contains the following updates:

Package	Update	Change
Flask (changelog)	patch	==3.1.0 -> ==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

v3.1.1

Compare Source

Released 2025-05-13

• Fix signing key selection order when key rotation is enabled via
  SECRET_KEY_FALLBACKS. :ghsa:4grg-w6v8-c28g
• Fix type hint for cli_runner.invoke. :issue:5645
• flask --help loads the app and plugins first to make sure all commands
  are shown. :issue:5673
• Mark sans-io base class as being able to handle views that return
  AsyncIterable. This is not accurate for Flask, but makes typing easier
  for Quart. :pr:5659

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