Fix clang-tidy: Prefer using over typedef

Author: rhaschke

include/pybind11/eigen.h

@@ -539,9 +539,9 @@ struct type_caster<Type, enable_if_t<is_eigen_other<Type>::value>> {
 
 template<typename Type>
 struct type_caster<Type, enable_if_t<is_eigen_sparse<Type>::value>> {
-    typedef typename Type::Scalar Scalar;
-    typedef remove_reference_t<decltype(*std::declval<Type>().outerIndexPtr())> StorageIndex;
-    typedef typename Type::Index Index;
+    using Scalar = typename Type::Scalar;
+    using StorageIndex = remove_reference_t<decltype(*std::declval<Type>().outerIndexPtr())>;
+    using Index = typename Type::Index;
     static constexpr bool rowMajor = Type::IsRowMajor;
 
     bool load(handle src, bool) {