Enforce node version on python (#888)

### Description

We have to make sure that the Python examples stick to the `18.x`
runtime until Python is supported by the new build container.
See: https://vercel.com/docs/deployments/build-image

### Demo URL

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### 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?

python/django/package.json

@@ -0,0 +1,5 @@
+{
+  "engines": {
+    "node": "18.x"
+  }
+}

python/flask2/package.json

@@ -0,0 +1,5 @@
+{
+  "engines": {
+    "node": "18.x"
+  }
+}

python/flask3/package.json

@@ -0,0 +1,5 @@
+{
+  "engines": {
+    "node": "18.x"
+  }
+}

python/hello-world/package.json

@@ -0,0 +1,5 @@
+{
+  "engines": {
+    "node": "18.x"
+  }
+}

python/nextjs-flask/package.json

@@ -24,5 +24,8 @@
     "react-dom": "18.2.0",
     "tailwindcss": "3.3.2",
     "typescript": "5.0.4"
+  },
+  "engines": {
+    "node": "18.x"
   }
 }