Bump flask from 2.3.3 to 3.1.3 (#744)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements.txt

@@ -3,7 +3,7 @@ Lektor==3.3.12
 # These are pinned because they're versions that are known to work
 Babel==2.17.0
 EXIFRead==3.0.0
-Flask==2.3.3
+Flask==3.1.3
 Jinja2==3.1.6
 Lektor==3.3.12
 Werkzeug==2.3.8