title: Django URLField doesn't accept hostname-only URLs
body: <p>I'm currently using Django with Docker. I have main dispatcher Django project, and several other microservices that are running on different Docker containers.</p>
  
  <p>When I start up a new microservice, I want to add new Django model which has <code>URLField</code> pointing that microservice. But <code>URLField</code> doesn't accept such URLs, e.g. <a href="http://foo-service/" rel="nofollow noreferrer">http://foo-service/</a> and it's because of <a href="https://github.com/django/django/blob/46b3e3ffdc25b5a42762df20e536401fc512c5f3/django/core/validators.py#L74" rel="nofollow noreferrer">these code</a>.</p>
  
  <p>Of course I could define custom validator or custom field that inherits <code>URLField</code>, but shouldn't <code>URLField</code> accept those URLs?</p>