fix flask example (#1252)

### Description

tiny bugfix in flask example + gitignore

### Demo URL

### Type of Change

- [ ] New Example
- [x] Example updates (Bug fixes, new features, etc.)
- [ ] Other (changes to the codebase, but not to examples)

### New Example Checklist

- [ ] 🛫 `npm run new-example` was used to create the example
- [ ] 📚 The template wasn't used but I carefuly read the [Adding a new
example](https://github.com/vercel/examples#adding-a-new-example) steps
and implemented them in the example
- [ ] 📱 Is it responsive? Are mobile and tablets considered?

Author: ricardo-agz

python/flask/.gitignore

@@ -0,0 +1,13 @@
+.vercel
+*.log
+*.pyc
+__pycache__
+
+# Environments 
+.env 
+.venv 
+env/ 
+venv/ 
+ENV/ 
+env.bak/ 
+venv.bak/ 

python/flask/pyproject.toml

@@ -3,6 +3,6 @@ name = "vercel-flask-starter"
 version = "0.1.0"
 requires-python = ">=3.9"
 dependencies = [
-  "flask~=3.1",
-  "gunicorn~=23.0.0
+  "flask>=3.1,<4.0",
+  "gunicorn~=23.0.0"
 ]