TST: test_quantity_ufuncs.py fails when numpy-dev is used without scipy

[pllim]

Example log: https://github.com/astropy/astropy/actions/runs/8025362924/job/21925825752

FAILED units/tests/test_quantity_ufuncs.py::TestUfuncHelpers::test_coverage -
AssertionError: assert {<ufunc 'isti...unc 'rindex'>} == set()
  
  Extra items in the left set:
  <ufunc 'istitle'>
  <ufunc 'isalnum'>
  <ufunc 'index'>
  <ufunc 'islower'>
  <ufunc 'isupper'>
  <ufunc 'rindex'>
  
  Full diff:
  - set()
  + {
  +     <ufunc 'index'>,
  +     <ufunc 'isalnum'>,
  +     <ufunc 'islower'>,
  +     <ufunc 'istitle'>,
  +     <ufunc 'isupper'>,
  +     <ufunc 'rindex'>,
  + }

@mhvk and @neutrinoceros , you have been on top of this lately, do you have idea how to fix? Thanks!

p.s. Thanks to @dhomeier's foresight to ask for this combo!

[dhomeier]

numpy/numpy#25791; should probably be added to UNSUPPORTED_UFUNCS in helpers.py following suite in #15525.

[mhvk]

Indeed, they are string ufuncs that Quantity does not support - but should be OK with Masked