Allow URLValidator to skip schemes validation

[Tim Graham]

As proposed by Claude in #26418, "provide some special value for scheme, like '__all__', to allow for all possible schemes (in the same spirit as the ModelForm fields / exclude attributes).

[Claude Paroz]

It's not exactly about skipping scheme validation, but a minimal validation (e.g. all ascii letters and maximal length).

[Tim Graham]

​PR with some comments for improvement.

[Baptiste Mispelon]

I took Burhan Khalid's pull request and updated it to target the 3.1 release.

I also tweaked the regexp for the accepted schemes to better match the RFC (I think).
As suggested by Tim in the previous pull request, I added a test to make sure all currently assigned schemes [1] are validated by the regexp.

​PR here

[1] ​https://www.iana.org/assignments/uri-schemes/uri-schemes.xhtml

[Carlton Gibson]

As I commented on the PR, I'm not 100% convinced we shouldn't be saying that folks should create a custom validator for the known set of schemes that they want to accept, rather than adding an '__all__'. What do others think?

[felixxm]

Agreed, I don't see a wide usage of __all__. In most of cases you want to add some schemas which is ​already possible. I believe solving #25594 would be a real solution for easy customizing the list of schemes.