• Tests added
• Passes isort -rc . && black . && mypy . && flake8
• Fully documented, including whats-new.rst for all changes and api.rst for new API

This adds a days_in_month accessor to CFTimeIndex, which allows for easy computation of monthly time weights for non-standard calendars:

In [1]: import xarray as xr

In [2]: times = xr.cftime_range("2000", periods=24, freq="MS", calendar="noleap")

In [3]: da = xr.DataArray(times, dims=["time"])

In [4]: da.dt.days_in_month
Out[4]:
<xarray.DataArray 'days_in_month' (time: 24)>
array([31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31, 31, 28, 31, 30, 31,
       30, 31, 31, 30, 31, 30, 31])
Coordinates:
  * time     (time) object 2000-01-01 00:00:00 ... 2001-12-01 00:00:00

This simplifies the "Calculating Seasonal Averages from Timeseries of Monthly Means" example @jhamman wrote for the docs a while back, which I've taken the liberty of updating.

The ability to add this feature to xarray is thanks in large part to @huard, who added a daysinmonth attribute to cftime.datetime objects late last year: Unidata/cftime#138.