Ensures that the environment is good for unicode on Python 3.

def _verify_python3_env():
    """Ensures that the environment is good for unicode on Python 3."""
    if PY2:
        return
    try:
        import locale
        fs_enc = codecs.lookup(locale.getpreferredencoding()).name
    except Exception:
        fs_enc = 'ascii'
    if fs_enc != 'ascii':
        return

    extra = ''
    if os.name == 'posix':
        import subprocess
        try:
            rv = subprocess.Popen(['locale', '-a'], stdout=subprocess.PIPE,
                                  stderr=subprocess.PIPE).communicate()[0]
        except OSError:
            rv = b''
        good_locales = set()
        has_c_utf8 = False

        # Make sure we're operating on text here.
        if isinstance(rv, bytes):
            rv = rv.decode('ascii', 'replace')

        for line in rv.splitlines():
            locale = line.strip()
            if locale.lower().endswith(('.utf-8', '.utf8')):
                good_locales.add(locale)
                if locale.lower() in ('c.utf8', 'c.utf-8'):
                    has_c_utf8 = True

        extra += '\n\n'
        if not good_locales:
            extra += (
                'Additional information: on this system no suitable UTF-8\n'
                'locales were discovered.  This most likely requires resolving\n'
                'by reconfiguring the locale system.'
            )
        elif has_c_utf8:
            extra += (
                'This system supports the C.UTF-8 locale which is recommended.\n'
                'You might be able to resolve your issue by exporting the\n'
                'following environment variables:\n\n'
                '    export LC_ALL=C.UTF-8\n'
                '    export LANG=C.UTF-8'
            )
        else:
            extra += (
                'This system lists a couple of UTF-8 supporting locales that\n'
                'you can pick from.  The following suitable locales were\n'
                'discovered: %s'
            ) % ', '.join(sorted(good_locales))

        bad_locale = None
        for locale in os.environ.get('LC_ALL'), os.environ.get('LANG'):
            if locale and locale.lower().endswith(('.utf-8', '.utf8')):
                bad_locale = locale
            if locale is not None:
                break
        if bad_locale is not None:
            extra += (
                '\n\nClick discovered that you exported a UTF-8 locale\n'
                'but the locale system could not pick up from it because\n'
                'it does not exist.  The exported locale is "%s" but it\n'
                'is not supported'
            ) % bad_locale

    raise RuntimeError(
        'Click will abort further execution because Python 3 was'
        ' configured to use ASCII as encoding for the environment.'
        ' Consult https://click.palletsprojects.com/en/7.x/python3/ for'
        ' mitigation steps.' + extra
    )