Solve missing interpolation method (cubicspline)
By commit 8bb2cc1 scipy.interpolate.CubicSpline method is
referenced in the pandas documentation (see pandas/core/generic.py)
but it is not wrapped by any interpolation method. This PR solves
this by adding the corresponding wrapper. SciPy's CubicSpline
is a cubic spline data interpolator that allows explicit control
of the boundary conditions for the interval.

Changes to be committed:
	modified:   pandas/core/missing.py
	modified:   pandas/tests/series/methods/test_interpolate.py

- [x] closes #xxxx (no issue opened but PR #20270 commented instead)
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

thanks @khyox very nice Many thanks @jreback and @mroeschke for your guidance on my 1st PR to the awesome pandas package!