BUG: fix CategoricalBlock unpickling

TST: add categorical frame and series to generate_legacy_pickles

TST: generate legacy pickle including frames/series with categoricals
for 0.15.0

COMPAT: ensure pre-0.15.0 pickles of Categorical are read

TST: move assert_categorical_equal to tm.TestCase
Merge pull request #8519 from immerrr/fix-categoricalblock-pickling

BUG: fix CategoricalBlock pickling