fix(stattests): silence scipy>=1.17 anderson_ksamp midrank deprecation (#1534)

[jbbqqf]

Summary

Closes #1534.

scipy 1.17 deprecated the implicit `midrank` default of `anderson_ksamp` in
favour of an explicit `variant=` keyword. Calling
`anderson_ksamp(samples)` without `variant=` emits a `DeprecationWarning`
per call (visible in any test run, see the "warnings summary" section of
`pytest tests/stattests/` on `origin/main`):

`Parameter 'variant' has been introduced to replace 'midrank';
'midrank' will be removed in SciPy 1.19.0. Specify 'variant' to silence
this warning. Note that the returned object will no longer be unpackable
as a tuple, and 'critical_values' will be omitted.`

Issue #1534 reports that this breaks user CI policies that promote
`DeprecationWarning` to errors.

Fix

In `src/evidently/legacy/calculations/stattests/anderson_darling_stattest.py`:

• On scipy>=1.17, call `anderson_ksamp(samples, variant="midrank")` — the
  string matches the legacy `midrank=True` default — and read the result
  object's `.pvalue` attribute.
• On older scipy (down to 1.10, the project's floor), keep the legacy
  3-tuple unpacking.

Reproduce BEFORE/AFTER yourself (copy-paste)

```bash
git fetch origin && git fetch https://github.com/jbbqqf/evidently.git fix/1534-scipy-anderson-deprecation:_pr1534
pip install -q -e ".[dev]" >/dev/null

run_check() {
python - <<'PY'
import warnings, pandas as pd
from evidently.legacy.calculations.stattests.anderson_darling_stattest import anderson_darling_test
ref = pd.Series([38.7, 41.5, 43.8, 44.5, 45.5, 46.0, 47.7, 58.0])
cur = pd.Series([39.2, 39.3, 39.7, 41.4, 41.8, 42.9, 43.3, 45.8])
with warnings.catch_warnings(record=True) as w:
warnings.simplefilter("always")
anderson_darling_test.func(ref, cur, "num", 0.001)
hits = [x for x in w if 'midrank' in str(x.message) or 'variant' in str(x.message)]
print(f"midrank/variant warnings: {len(hits)}")
for h in hits:
print(" -", h.category.name, str(h.message)[:120])
PY
}

BEFORE — origin/main: scipy emits DeprecationWarning.

git checkout origin/main -- src/evidently/legacy/calculations/stattests/anderson_darling_stattest.py
run_check

Expected (scipy>=1.17): 'midrank/variant warnings: 1' with the deprecation message.

AFTER — this branch: silenced.

git checkout _pr1534 -- src/evidently/legacy/calculations/stattests/anderson_darling_stattest.py
run_check

Expected: 'midrank/variant warnings: 0'.

Restore.

git checkout origin/main -- src/evidently/legacy/calculations/stattests/anderson_darling_stattest.py
```

What I ran locally

• `pytest tests/stattests -q` -> 63 passed
• `pytest tests/stattests/test_stattests.py::test_anderson_darling_no_scipy_deprecation_warning`
  fails on `origin/main` (DeprecationWarning emitted), passes on this branch.
• `ruff check` and `ruff format --check` -> clean.

Edge cases

scipy version	Path taken	Numerical result
1.17+	`variant="midrank"` + `.pvalue`	Same p-value as legacy midrank=True
1.10 .. 1.16	Legacy 3-tuple unpack	Unchanged
<1.10	Below pyproject floor; not supported	n/a

AI disclosure

This pull request was authored with assistance from Anthropic's Claude (an AI
coding assistant) running under my direction. I verified the SciPy
deprecation message and the new return-shape contract via `help(anderson_ksamp)`
on scipy 1.17 before writing the patch.