weightstats quantile error with len 1 array

[brl0]

Describe the bug

DescrStatsW.quantile throws an error when the inputs contain one element.
TypeError: len() of unsized object

This seems to happen because the weights array is squeezed to a zero-dimensional scalar when it contains only a single value.

This occurred while I was performing a groupby/apply operation on a pandas dataframe in which the groups may contain one or more rows.

I will submit a PR with a fix and a simple test for the issue.

Code Sample, a copy-pastable example if possible

from statsmodels.stats.weightstats import DescrStatsW
x1 = [1]
w1 = [1]
d1 = DescrStatsW(x1, w1)
assert (d1.quantile([0.0, 0.5, 1.0]) == 1).all()

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Expected Output

A clear and concise description of what you expected to happen.

Output of import statsmodels.api as sm; sm.show_versions()

[paste the output of import statsmodels.api as sm; sm.show_versions() here below this line]
ImportError: cannot import name '_initialization' from 'statsmodels.tsa.statespace' (/home/brl0/repos/statsmodels/statsmodels/tsa/statespace/__init__.py)

[josef-pkt]

duplicate of #4282

but we leave it open to fix this soon

[josef-pkt]

fixed in #7230 which is rebased #7047