TST: Adjust boxplot tests following MPL API change

fliers on a boxplot are now 1 object instead of 2.
Remove DataFrame.delevel
Merge pull request #8451 from jsexauer/fix_delevel

Remove DataFrame.delevel
BUG: reset identity on legacy index pickles (GH8431)
Merge pull request #8454 from jreback/legacy_pickle

BUG: reset identity on legacy index pickles (GH8431)
DOC/CLN: Cleanups plotting.py
PERF: Improve performance of CustomBusinessDay
TST: compat for *.pkl -> *.pickle in pandas/tests/data
PEP8 fixups
Merge pull request #8293 from bjonen/fix_8236_2

PERF: Improve performance CustmBusinessDay - 2nd
TST: fix path for getting pickles wrt (GH8431)
Merge pull request #8429 from TomAugspurger/flier-test-adjust

TST: Adjust boxplot tests following MPL API change
PERF: optimize storage type for codes in Categoricals (GH8453)
type diversity breaks alignment
BUG: fix applymap to handle Timedelta
Merge pull request #8459 from TomAugspurger/plot_cln

VIS: Cleanups in plotting.py
DOC: Cleanup of plots in visualization.py

DOC: Use square figsize for pie plots
DOC: Don't share axis labels and reorder to fit in page.
Merge pull request #8461 from jreback/applymap

BUG: fix applymap to handle Timedelta
Merge pull request #8455 from jreback/cat2

PERF: optimize storage type for codes in Categoricals (GH8453)
Merge pull request #8457 from behzadnouri/align-rhs

BUG: type diversity breaks alignment