Handle non text in style/secho by converting to text

[segevfiner]

Call str() on any non-string type. echo() also supports printing bytes, but this PR doesn't support that, ending up printing its repr (b'bytes'). I'm not sure how to handle it, since making style() support bytes might be rather ugly.

[davidism]

Within echo, there's already a check to not try to strip bytes of color information. This makes sense, as it wouldn't make sense to color raw bytes being output (even if they do represent text). So I think it makes sense to only do str(message).

That said, I did try an implementation for decoding bytes passed to secho, but decided against it. It had to be in secho and not style since only secho has information about the stream being written to.

from ._compat import get_best_encoding

# extracted from echo for use in secho as well
def _get_echo_file(file, err):
    if file is not None:
        return file

    if err:
        return _default_text_stderr()
    else:
        return _default_text_stdout()

def secho(message, file=None, err=False, ...):
    file = _get_echo_file(file, err)

    if message is not None:
        if is_bytes(message):
            file = _get_echo_file(file, err)
            encoding = get_best_encoding(file)
            message = message.decode(encoding)
        elif not isinstance(message, str):
            message = str(message)

        message = style(message, **styles)

    return echo(message, file=file, err=err, ...)

This isn't completely correct, it doesn't pull out the errors attribute of the stream for decoding. In the end, this seemed a bit too complicated. Since 8.0 is Python 3-only, it also makes more sense to require text for output, requiring users to explicitly decode bytes being passed is the common pattern now.

[davidism]

I don't think this addresses #944, that seems to be about allowing ClickException.show() to use secho() instead of echo(), not being able to pass other types to secho().

[davidism]

I guess we could also do not is_bytes(message) before applying style() in secho(). That way bytes get passed directly to echo().