BUG: fix weightstats quantile for len 1 array

[brl0]

• closes weightstats quantile error with len 1 array #7046
• tests added / passed.
• code/documentation is well formatted.
• properly formatted commit message. See
  NumPy's guide.

Notes:

• It is essential that you add a test when making code changes. Tests are not
  needed for doc changes.
• When adding a new function, test values should usually be verified in another package (e.g., R/SAS/Stata).
• When fixing a bug, you must add a test that would produce the bug in master and
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  git diff upstream/master -u -- "*.py" | flake8 --diff --isolated
  

  assuming flake8 is installed. This command is also available on Windows
  using the Windows System for Linux once flake8 is installed in the
  local Linux environment. While passing this test is not required, it is good practice and it help
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[coveralls]

Coverage increased (+0.002%) to 88.356% when pulling 1a10419 on brl0:patch-1 into df1c1a1 on statsmodels:master.

[josef-pkt]

rebased version in #7230 has been merged

@brl0 thanks for the PR and fix