BUG: Bug in .xs with a nan in level when dropped (GH6574)
Merge pull request #6579 from jreback/xs

BUG: Bug in .xs with a nan in level when dropped (GH6574)
CLN: Finish changing assert_(...) to specialized forms

Finishes #6175, to the extent that everything remaining can be mapped to assertTrue or assertFalse, unless larger refactorings are desired.
Merge pull request #6580 from bwignall/assert_6175_cleanup

CLN: Finish changing assert_(...) to specialized forms