`_preformat_levels` in `utils/loggable.py` assumes that the fields in its `TerminalController` are uniformly `bytes` or uniformly `string`s

In _preformat_levels, which builds the list _FORMATTED_LEVELS, the code may need to convert up to three TerminalController fields from bytes to strs on each iteration:

• terminal_controller.BOLD,
• getattr(terminal_controller, COLORS[level]), and
• terminal_controller.NORMAL.

The current implementation, however, only checks one field's type, and then assumes that the other fields have the same type:

if isinstance(terminal_controller.BOLD, bytes):
    formatter = ''.join(
        (terminal_controller.BOLD.decode(),
         getattr(terminal_controller, COLORS[level]).decode(),
         log_level_name(level),
         terminal_controller.NORMAL.decode()))
else:
    formatter = ''.join(
        (terminal_controller.BOLD,
         getattr(terminal_controller, COLORS[level]),
         log_level_name(level),
         terminal_controller.NORMAL))

In some environvents (such as my own), this leads to a crash when terminal_controller.BOLD has type bytes, but getattr(terminal_controller, COLORS[level]) has type str, as Python 3 strings have no decode method to call.