Grokcore.message

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- Nothing changed yet.

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- Drop support for Python 2.7, 3.4, 3.5, 3.6.

- Add support for Python 3.7, 3.8, 3.9, 3.10, 3.11.

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- Replace the use of `grok.implements()` with the `grok.implementer()`
directive throughout.

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- Python 3 compatibility.

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- Update tests.

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- Tests fixed by explicitely registering the IClientIdManager and
ISessionDataContainer utilities. The ftesting.zcml was re-introduced for this.