Bump flask from 0.12.1 to 2.3.2 in /lib

Bumps [flask](https://github.com/pallets/flask) from 0.12.1 to 2.3.2.
- [Release notes](https://github.com/pallets/flask/releases)
- [Changelog](https://github.com/pallets/flask/blob/main/CHANGES.rst)
- [Commits](pallets/flask@0.12.1...2.3.2)

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

Signed-off-by: dependabot[bot] <support@github.com>

lib/requirements.txt

@@ -2,7 +2,7 @@
 Pillow==4.1.1
 
 # Flask takes care of the HTTP.
-Flask==0.12.1
+Flask==2.3.2
 
 # Gunicorn hooks Flask up to Nginx for threads.
 gunicorn==19.7.1