CLN: Python 3.4 deprecation of assert_

#7131: replace instances of assert_(not b) with assertFalse(b) and some instances of assert_(b) with assertTrue(b). Work-in-progress
BUG: PeriodIndex slices with out of bounds results incorrect
Merge pull request #7161 from sinhrks/pslice2

BUG: PeriodIndex slices with out of bounds results incorrect
Merge pull request #7162 from bwignall/issue7131_assert

CLN: Python 3.4 deprecation of assert_