Fix build with Python 3.14

Index: python/mozbuild/mozbuild/frontend/reader.py
--- python/mozbuild/mozbuild/frontend/reader.py.orig
+++ python/mozbuild/mozbuild/frontend/reader.py
@@ -471,7 +471,7 @@ class TemplateFunction(object):
 
             return c(ast.Subscript(
                 value=c(ast.Name(id=self._global_name, ctx=ast.Load())),
-                slice=c(ast.Index(value=c(ast.Str(s=node.id)))),
+                slice=c(ast.Index(value=c(ast.Constant(value=node.id)))),
                 ctx=node.ctx
             ))
 
@@ -982,8 +982,8 @@ class BuildReader(object):
             key = None
             if isinstance(target, ast.Subscript):
                 assert isinstance(target.slice, ast.Index)
-                assert isinstance(target.slice.value, ast.Str)
-                key = target.slice.value.s
+                assert isinstance(target.slice.value, ast.Constant)
+                key = target.slice.value.value
 
             return name, key
 
@@ -991,11 +991,11 @@ class BuildReader(object):
             value = node.value
             if isinstance(value, ast.List):
                 for v in value.elts:
-                    assert isinstance(v, ast.Str)
-                    yield v.s
+                    assert isinstance(v, ast.Constant)
+                    yield v.value
             else:
-                assert isinstance(value, ast.Str)
-                yield value.s
+                assert isinstance(value, ast.Constant)
+                yield value.value
 
         assignments = []
 
