Update flask requirement from ==3.0.* to ==3.1.*

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@3.0.0...3.1.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

src/requirements.txt

@@ -1,7 +1,7 @@
 Flask-Mail==0.10.0
 flask-session==0.8.0
 Flask-WTF==1.2.1
-Flask==3.0.*
+Flask==3.1.*
 Werkzeug==3.0.*
 cs50==9.4.0
 pyjwt>=2.8.0