And let pybstr/pyustr point to version of bstr/ustr types that is actually in use:
- when bytes/unicode are not patched -> to _pybstr/_pyustr
- when bytes/unicode will be patched -> to bytes/unicode to where original
  _pybstr/_pyustr were copied during bytes/unicode patching.
at runtime the code uses pybstr/pyustr instead of _pybstr/_pyustr.