[2.7] Allow 3.0 requirements

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | lets see
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

2.7 is going to provide almost the same interfaces as 3.0 but also a BC layer for previous releases.
This means that 2.7 is going to be mostly compatible with 3.0.
Lets continuously test this assumption and see when and were we break compat with 2.7 (and try to avoid it as much as possible to smoothen the transition to 3.0).
I would not do it for the Form component, because we already know that we will have hard BC breaks in it (changing the typehint to use OptionsResolver rather than the deprecated OptionsResolverInterface)