Do we need to explicitly document the return value change of _file which is documented with a separate versionchanged directive for 3.7 and 3.8? Code like below could fail in them since TextIOWrapper doesn't have getvalue attribute as the minor version upgrade is done.

import tempfile

with tempfile.SpooledTemporaryFile(mode='wt') as f:
    f.write('abc')
    assert f._file.getvalue() == 'abc'