models.URLField does not validate with an rtmp:// url

[yeah]

models.URLField does not validate with an rtmp:// url

checking the source, it is validated twice

class URLField(CharField):
    default_validators = [validators.URLValidator()]
    description = _("URL")

And

    def formfield(self, **kwargs):
        # As with CharField, this will cause URL validation to be performed
        # twice.
        defaults = {
            'form_class': forms.URLField,
        }

in django.core.validators.UrlValidator we find at least one of the culprits:

    schemes = ['http', 'https', 'ftp', 'ftps']

RFC 3986 (I believe retained for RFC7320) defines scheme:

3.1. Scheme

Each URI begins with a scheme name that refers to a specification for
assigning identifiers within that scheme. As such, the URI syntax is
a federated and extensible naming system wherein each scheme's
specification may further restrict the syntax and semantics of
identifiers using that scheme.

Scheme names consist of a sequence of characters beginning with a
letter and followed by any combination of letters, digits, plus
("+"), period ("."), or hyphen ("-"). Although schemes are case-
insensitive, the canonical form is lowercase and documents that
specify schemes must do so with lowercase letters. An implementation
should accept uppercase letters as equivalent to lowercase in scheme
names (e.g., allow "HTTP" as well as "http") for the sake of
robustness but should only produce lowercase scheme names for
consistency.

scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )

[Tim Graham]

It's not entirely clear what your proposal is, however, changing the default list of accepted schemes would be backwards-incompatible as described in #25593.

I believe solving #25594 would ease customizing the list of schemes and address the concern of this ticket.