[bashtage]

Follow-up to #1016, from its review:

• Docstring: the date_index entry of date_to_index lists Series: "DatetimeIndex, Series or ndarray".
• Errors: they describe date_index as it was passed. Since BUG: Accept timezone-aware date Series #1016, a Series is converted to a DatetimeIndex before the checks, so an empty or unsorted Series was reported as shape (3,) (DatetimeIndex). It is now reported as (Series). The converted dates go into a separate variable, and _describe is called on the input.
• Release note: adds a 9.0 note in Bug fixes for BUG: Accept timezone-aware date Series #1016, which was merged without one.

Checks

• New test: test_date_to_index_error_describes_input covers an unsorted, a duplicated and an empty date index, as a Series and as a DatetimeIndex, with and without a time zone (12 cases). The 6 Series cases fail on main, and the 6 DatetimeIndex cases pass on both.
• Test runs:
  • the utility tests and test_docstrings.py pass (1,927);
  • the univariate and utility tests that use dates, cutoffs, append or forecast pass (6,987).
• Lint: ruff (lint and format), black, isort and flake8 pass, numpydoc lint passes, and mypy reports the same errors as on main.
• Release note: the new lines are at most 88 columns and ASCII, and docutils parses 9.0.rst with no warnings.

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