Find text I/O that relies on the default encoding

Python 3.15 uses UTF-8 as the default text encoding whatever the locale (PEP 686). Marks text-mode open(), io.open(), pathlib.Path.open(), read_text(), write_text(), io.TextIOWrapper(), and subprocess calls with text=True or universal_newlines=True that pass no encoding. Pass encoding="utf-8", or encoding="locale" to keep the previous behaviour. A call whose mode is not a literal is not reported.