DOC: CONTRIBUTING.md: Gold plating: syntax, punctuation, Markdown
BUG: Fix bound checking for Timestamp() with dt64 #4065

To fix the bug, this change adds bounds checking to
_get_datetime64_nanos() for numpy datetimes that aren't already in [ns]
units.

Additionally, it updates _check_dts_bounds() to do the bound check just
based off the pandas_datetimestruct, by comparing to the minimum and
maximum valid pandas_datetimestructs for datetime64[ns].  It is simpler
and more accurate than the previous system.

Also includes a number of small refactors/fixes to deal with new error
cases that didn't exist when invalid datetime64s were just silently
coerced into the valid datetime64[ns] range.
BUG: MultiIndex.get_level_values() replaces NA by another value (#5074)
Make _na_value a class attribute.
Merge pull request #5090 from goyodiaz/multiindex-nan

BUG: MultiIndex.get_level_values() replaces NA by another value (#5074)
DOC: revised io.rst
Merge branch 'timestamp-object-bounds' of https://github.com/danbirken/pandas into danbirken-timestamp-object-bounds

Conflicts:
	doc/source/release.rst
DOC: timeseries.rst updates
TST: use default utf8 encoding when passing encoding to parser as Bytes (GH5141)
Merge pull request #5149 from jreback/sparc_fix3

TST: use default utf8 encoding when passing encoding to parser as Bytes (GH5141)
TST/CI/BUG: fix borked call to read_html testing code
Merge pull request #5151 from cpcloud/read-html-lxml-skips

TST/CI/BUG: fix borked call to read_html testing code
DOC: 5143 (read_html testing bug) release notes
DOC: minor doc correction (issue number) from the previous commit
Merge pull request #5086 from westurner/patch-2

DOC: CONTRIBUTING.md: Gold plating: syntax, punctuation, Markdown format...
BUG: convert replacement tuples to lists

This fixes a bug that makes it appear as if replace is incorrectly using
regular expressions.
Merge pull request #5145 from cpcloud/replace-exact-fix

BUG: allow tuples in recursive call to replace
BUG: Fix to_datetime() uncaught error with unparseable inputs #4928
API/CLN: provide isnull/notnull on NDFrame objects
PERF: remove auto-boxing on isnull/notnull
Merge pull request #5154 from jreback/perf

PERF: remove auto-boxing on isnull/notnull