update requirements to use current Flask (3.1.0)

Other dependencies arise from that (did a pip freeze after
loading Flask 3.1.0). The only dependency that has been
dropped is zipp, and I don't think it's a requirement (any
more)

requirements.txt

-click==8.1.4
-Flask==2.3.2
-importlib-metadata==6.8.0
-itsdangerous==2.1.2
-Jinja2==3.1.2
-MarkupSafe==2.1.3
-Werkzeug==2.3.6
-zipp==3.16.0
-
+blinker==1.9.0
+click==8.1.8
+Flask==3.1.0
+itsdangerous==2.2.0
+Jinja2==3.1.6
+MarkupSafe==3.0.2
+Werkzeug==3.1.3