Investigate test_median_clean failures

[saimn]

test_median_clean is occasionally failing on some architectures: #71 (mipsel), #82 (windows wheels), #66 (comment) (i686), #85 (cp311-win_amd64 wheel):

Was marked as xfail in #85.

      def test_median_clean(testdata):
          imdata, crmask = testdata
          # Because our image only contains single cosmics, turn off
          # neighbor detection. Also, our cosmic rays are high enough
          # contrast that we can turn our detection threshold up.
          _mask, clean = detect_cosmics(imdata, readnoise=10., gain=1.0,
                                        sigclip=6, sigfrac=1.0, cleantype='median')
      
          assert (clean[crmask] != imdata[crmask]).sum() == crmask.sum()
      
          # Run it again on the clean data. We shouldn't find any new cosmic rays
          _mask2, _clean2 = detect_cosmics(clean, readnoise=10., gain=1.0,
                                           sigclip=6, sigfrac=1.0, cleantype='median')
  >       assert _mask2.sum() == 0
  E       assert 100 == 0