Fix error when compiling with clang

csv.cpp:83:29: error: non-constant-expression cannot be narrowed from type 'Py_ssize_t' (aka 'long') to 'std::basic_string_view::size_type' (aka 'unsigned long') in initializer list [-Wc++11-narrowing] app.read_stream({p, n}); ^ csv.cpp:83:29: note: insert an explicit cast to silence this issue app.read_stream({p, n}); ^ static_cast<size_type>( ) 1 error generated.