a method to validate heroku is installed

def _validate_install(self):

        ''' a method to validate heroku is installed '''

        self.printer('Checking heroku installation ... ', flush=True)
    
    # import dependencies
        from os import devnull
        from subprocess import call, check_output
        
    # validate cli installation
        sys_command = 'heroku --version'
        try:
            call(sys_command, shell=True, stdout=open(devnull, 'wb'))
        except Exception as err:
            self.printer('ERROR')
            raise Exception('"heroku cli" not installed. GoTo: https://devcenter.heroku.com/articles/heroku-cli')
    
    # print response and return
        self.printer('done.')
        
        return True