Cftime

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* num2date uses 'proleptic_gregorian' scheme when basedate is post-Gregorian but date is pre-Gregorian
(issue 182).
* fix 1.2.0 regression (date2num no longer works with numpy scalar array inputs, issue 185).
* Fix for issue 187 (have date2num round to the nearest second when within 1
microsecond).
* Fix for issue 189 (leap years calculated incorrectly for negative years in
proleptic_gregorian calendar).

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* Return the default values of dayofwk and dayofyr when calendar
is '' (issue 173).
* fix treatment of masked arrays in num2date and date2num (issue 175).
Also make sure masked arrays are output from num2date/date2num if
masked arrays are input.
* Where possible, use timedelta arithmetic to decode times exactly within
num2date (issue 171).
* Make taking the difference between two cftime datetimes to produce a
timedelta exact to the microsecond; depending on the units encoding,
this enables date2num to be exact as well (issue 109).
* utime.date2num/utime.num2date now just call module level functions.
JulianDayFromDate/DateFromJulianDay no longer used internally (PR 180).

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* add isoformat method for compatibility with python datetime (issue 152).
* make 'standard' default calendar for cftime.datetime
so that dayofwk,dayofyr methods don't fail (issue 169).

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* change dayofwk and dayofyr attributes into properties (issue 158)
* fix for issue 165 (python datetime should be returned when
only_use_cftime_datetimes=False).

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* include pyproject.toml in MANIFEST.in so it gets
included in source tarball (issue 154).

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* Fix error installing with pip on python 3.8 by following
PEP 517 (issue 148, PR 149)