Contains #1960 plus one more fix.
Fix lowess for constant inputs.
Correct fix for lowess; generative tests.
TST: Additional test from #1960
BUG: Avoid division by zero. Closes #1798.
See how this looks. Basically just another avoiding division by zero for the case where we only have one observation used in the weighted regression.
Coverage decreased (-0.0%) when pulling f28610e on jseabold:fix-1798 into da58f21 on statsmodels:master.
Backport PR #2105: BUG: Avoid returning nans in lowess.