avoid write permission requirement in FSStructureServer

modelling/pymod/_afdb_modelling.py

@@ -168,8 +168,9 @@ class FSStructureServer:
             tmp.sort()
             sorted_data_files = [x[1] for x in tmp]
             for f in sorted_data_files:
-                self._data_fh.append(open(f, 'r+b'))
-                self._data.append(mmap.mmap(self._data_fh[-1].fileno(), 0))
+                self._data_fh.append(open(f, 'rb'))
+                self._data.append(mmap.mmap(self._data_fh[-1].fileno(), 0,
+                                            prot=mmap.PROT_READ))
         return self._data
 
     @property