DOC: Draft the release notes for 7.1

[bashtage]

Add a "Version 7.1" section at the top of doc/source/changes/7.0.rst, the file that changes.rst includes. The file keeps its CRLF line endings, and the 7.0 section below the new one is unchanged.

The section follows the style of the earlier notes: short bullets that end with their issue or pull request links, under these headings: Highlights, New features, Changes that alter results, Backward-incompatible changes, Bug fixes, Documentation and maintenance, Compatibility, Deprecations and future changes, Minimum versions and Contributors. Changes that alter results has one bullet for each difference between 7.0 and main found by a comparison of about 7,000 fits, and gives the way back to the 7.0 behavior where there is one (IVGMMCUE: center=False).

It is a draft for the release steps to finish. It describes main (208845b) and the branches of the 7.1 plan that were verified but not merged when it was written: packaging, label alignment, the Hausman and Pesaran CD tests, the IV feature hardening, the exports, the deprecations and the AbsorbingLS and IVGMMCUE fixes. What is missing is marked by one-line RST comments that start with "TODO(R2)". They are not rendered. They mark the release date, the PR numbers, three open decisions (the series that FirstDifferenceOLS.pesaran_cd uses, the default kernel bandwidth of IVGMM and IVGMMCUE, and the reference when dependent has no labels), fixes that are not merged, the array_like coercion, the documentation and notebooks, and the wheels.

Checks

• Every :class:, :meth: and :attr: target exists (imported from main and the Hausman branch).
• A scratch Sphinx 9.1 project that builds changes.rst and the past releases with -n -W has no RST error or warning. The only warnings are the API references, which cannot resolve there because the API pages are not built, and the build passes when they are ignored.
• git diff has only added lines.

Notes

• The subsections are rubrics, not section titles. changes.rst has a "Past Releases" title with a title style of its own, and any extra heading level in the included file makes docutils report "Inconsistent title style" for that title. Underlining "Past Releases" like "Change Log" would allow real sections.
• Only NormalTestStatistic is described as new: InapplicableTestStatistic has been in the package since 2020.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.58%. Comparing base (208845b) to head (7f9c9c6).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #760   +/-   ##
=======================================
  Coverage   99.58%   99.58%           
=======================================
  Files         110      110           
  Lines       20328    20328           
  Branches     1600     1600           
=======================================
  Hits        20244    20244           
  Misses         31       31           
  Partials       53       53           

Flag	Coverage Δ
adder	99.57% <ø> (ø)
subtractor	99.57% <ø> (ø)

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