[2.7] Allow 3.0 requirements
minor #13135 [2.7] Allow 3.0 requirements (nicolas-grekas)

This PR was merged into the 2.7 branch.

Discussion
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[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).

Commits
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b369bb6 [2.7] Allow 3.0 requirements