Ensure py.typed file added to _aqt folder in wheel

Closes #2276

build/configure/src/aqt.rs

@@ -4,7 +4,7 @@
 use ninja_gen::{
     action::BuildAction,
     command::RunCommand,
-    copy::CopyFiles,
+    copy::{CopyFile, CopyFiles},
     glob, hashmap, inputs,
     node::{CompileSass, EsbuildScript, TypescriptCheck},
     python::{python_format, PythonTest},
@@ -90,6 +90,15 @@ fn build_generated_sources(build: &mut Build) -> Result<()> {
             },
         },
     )?;
+    // we need to add a py.typed file to the generated sources, or mypy
+    // will ignore them when used with the generated wheel
+    build.add(
+        "qt/aqt:py.typed",
+        CopyFile {
+            input: "qt/aqt/py.typed".into(),
+            output: "qt/_aqt/py.typed",
+        },
+    )?;
     Ok(())
 }