CLN: More 2/3 fixes
TST/ENH: Add assert_isinstance util to util/testing
CLN: Modernize pandas/tests + better type checks

Use assert_isinstance + replace type(a) == b with assert_isinstance.
Some 2/3 fixes of .sort() --> sorted
Better instance tests in io/tests
CLN: assert_(isinstance --> assert_isinstace
CLN: 2/3 fixes to util/decorators
CLN: better instance checks in scripts/
CLN: better type check in perf_HEAD
CLN: Use compatible long in io/stata