CLN: Deprecation of assert_
#7131: Convert more instances of self.assert_(not b) to self.assertFalse(b) and self.assert_(b) to self.assertTrue(b)

There are still a few instances of assert_ left to be converted: if an assertion has multiple lines, which may require reformatting, I did not yet touch it.
 after this prob won't be any assert_ left!
 thanks!
