Merge remote-tracking branch 'upstream/master'

* upstream/master: (76 commits)
  CategoricalDtype construction: actually use fastpath (pandas-dev#17891)
  DEPR: Deprecate tupleize_cols in to_csv (pandas-dev#17877)
  BUG: Fix wrong column selection in drop_duplicates when duplicate column names (pandas-dev#17879)
  DOC: Adding examples to update docstring (pandas-dev#16812) (pandas-dev#17859)
  TST: Skip if no openpyxl in test_excel (pandas-dev#17883)
  TST: Catch read_html slow test warning (pandas-dev#17874)
  flake8 cleanup (pandas-dev#17873)
  TST: remove moar warnings (pandas-dev#17872)
  ENH: tolerance now takes list-like argument for reindex and get_indexer. (pandas-dev#17367)
  ERR: Raise ValueError when week is passed in to_datetime format witho… (pandas-dev#17819)
  TST: remove some deprecation warnings (pandas-dev#17870)
  Refactor index-as-string groupby tests and fix spurious warning (Bug 17383) (pandas-dev#17843)
  BUG: merging with a boolean/int categorical column (pandas-dev#17841)
  DEPR: Deprecate read_csv arguments fully (pandas-dev#17865)
  BUG: to_json - prevent various segfault conditions (GH14256) (pandas-dev#17857)
  CLN: Use pandas.core.common for None checks (pandas-dev#17816)
  BUG: set tz on DTI from fixed format HDFStore (pandas-dev#17844)
  RLS: v0.21.0rc1
  Whatsnew cleanup (pandas-dev#17858)
  DEPR: Deprecate the convert parameter completely (pandas-dev#17831)
  ...

Author: Krzysztof Chomski