Remove deprecation warning for Batch.__len__ again.

[mauritsvanrees]

Calling this method is simly unavoidable.
When a page template does a tal:repeat over a batch, Products.PageTemplates calls list(batch) and len(batch), both of which invoke calling __len__. You can try this out with bin/zopepy:

$ bin/zopepy
>>> from plone.base.batch import Batch
>>> len(Batch(range(10), 3))
<console>:1: DeprecationWarning: Use length attribute instead of __len__
3
>>> list(Batch(range(10), 3))
<console>:1: DeprecationWarning: Use length attribute instead of __len__
[0, 1, 2]

The deprecation warning was removed in Plone 5.2.3, see plone/Products.CMFPlone#3176 But during the move of the Batch class to plone.base in Plone 6.0 it was added again. Compare:

• plone/Products.CMFPlone@2cc2515#diff-fa1e7e5e81879e8c4f4485cf8d8204eba6f2677cb9f4e29de4776b0c1f0d4d93L21
• 6e26a37#diff-316da888fe8d39c68d5e0aee055b3bbfa95d522b4b09fe56b7edb1289bd72f9fR28

[mister-roboto]

@mauritsvanrees thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[mauritsvanrees]

@jenkins-plone-org please run jobs