Fix signature of max_len_string

Author: kinow

pandas/_libs/writers.pyx

@@ -135,7 +135,7 @@ def max_len_string_array(pandas_string[:] arr) -> Py_ssize_t:
     return m
 
 
-def max_len_string(val):
+cdef inline Py_ssize_t max_len_string(object val):
     """ return the maximum size of a string"""
     cdef:
         Py_ssize_t l = 0