fix lemma dependency on local hypothesis

[affeldt-aist]

Motivation for this change

This is a bug fix. The lemmas fubini1a and fubini1b (beside being ill-named) depended on a local (strong) hypothesis instead of the intended (weaker) Let as they used to, certainly a consequence of the change of semantics of Let in Coq (I am pretty sure they were right in the first place).

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

- [ ] added corresponding documentation in the headers

Reference: How to document

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• PRs with disorganized commits are very likely to be squash-rebased.

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[affeldt-aist]

This is uncontroversial so I'll maybe merge if the CI is green.