As of WTForms 2.2 (June 2nd, 2018), fields now render the required attribute if they have a validator that sets the required flag, such as DataRequired and InputRequired. If for some reason you don't want to render the attribute, you can pass required=False. Or if you want to disable all browser validation, you can set the novalidate attribute in the form tag. In general you should prefer to leave browser validation enabled, because it prevents a request/response for simple validation, which is desirable.
You are passing a positional argument after keyword arguments, which is a syntax error. Instead, pass required=True, which will set a bare attribute on the tag. Check the flags on a field to see if a Required validator was set: field.flags.required is a boolean. Create a URLField rather than passing the type manually.