[tonghuaroot]

variance() and pvariance() now raise ValueError('inf or nan encountered in data') for data containing inf or nan, matching stdev() and
pstdev() (gh-140938).

_ss() returns the sum of squared deviations as a Fraction for all finite
data, and as a non-Fraction only for inf/nan input, so the guard is a
type check on that value. A type check rather than math.isfinite(float(ss))
avoids an OverflowError on huge but finite integer data.

test_gh_140938 moves from TestPStdev into VarianceStdevMixin, so it now
covers variance, pvariance, stdev and pstdev, and gains the negative
(-inf) case.

• Issue: statistics.variance() can return a negative result for data containing infinity #160034