CI workflows: tighten permissions (#633)

Author: Tinche

.github/workflows/main.yml

@@ -1,6 +1,8 @@
 ---
 name: CI
 
+permissions: {}
+
 on:
   push:
     branches: ["main"]

.github/workflows/pypi-package.yml

@@ -1,6 +1,8 @@
 ---
 name: Build & maybe upload PyPI package
 
+permissions: {}
+
 on:
   push:
     branches: [main]