Construct Decimal from YAML float encoding.

def float_constructor(loader, node):
    """Construct Decimal from YAML float encoding."""
    s = loader.construct_scalar(node)
    if s == '.inf':
        return Decimal('Infinity')
    elif s == '-.inf':
        return -Decimal('Infinity')
    elif s == '.nan':
        return Decimal('NaN')
    return Decimal(s)