Cftime

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

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* fix microsecond formatting issue, ensure identical results
computed for arrays and scales (issue 143, PR 146).

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* improved exceptions for time differences (issue 128, PR 131).
* fix intersphinx entries (issue 133, PR 133)
* make only_use_cftime_datetimes=True by default, so cftime datetime
instances are returned by default by num2date (instead of returning python
datetime instances where possible). Issue 136, PR 135.
* Add daysinmonth attribute (issue 137, PR 138).
* If only_use_python_datetimes=True and only_use_cftime_datetimes=False,
num2date only returns python datetime instances and raises an exception
if this is not possible. num2pydate convenience function added which just calls
num2date with only_use_python_datetimes=True and
only_use_cftime_datetimes=False.
Remove positive times check, raise ValueError if python datetime
tries to compute a date before MINYEAR (issue 134, PR 139)
* Fix for fractional seconds in reference date in units string (issue 140,
PR 141).

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* fix for issue 126 (date2num error when converting a DatetimeProlepticGregorian
object). PR 127.


* create a release branch ('vX.Y.Zrel'). In the release branch...
* make sure version number in cftime/_cftime.pyx is correct.
* update README.md as needed.
* commit and push all of the above changes.
* create a pull request for the release branch.
* After release branch has been merged, tag a release
% git tag -a vX.Y.Zrel -m "version X.Y.Z release"
% git push origin --tags
Then create a release on github from that tag.
* Update the BUILD_COMMIT variable in .travis.yml and appveyor.yml
in https://github.com/MacPython/cftime-wheels.
Push those changes to trigger the binary wheel builds (for
macos x, linux and windows).
* run check-manifest --verbose to check that source tarball will be OK.
* Download the binary wheels from wheels.scipy.org, put them in a
directory called 'upload'. Create a source tarball using
% python setup.py clean
% python setup.py clean_cython
% python setup.py sdist
Add the source tarball to the 'upload' directory.
* upload the release files to pypi with twine
(twine upload upload/*). This will require creating a ~/.pypirc
file with your pypi login credentials.