Define Series.plot and Series.hist in class definition
Avoid fragile circular import between core.series and `plotting._core`, `plotting._tools`, `plotting._converter`

Ends the saga of #16913, #16931.

In the status quo, the tail end of `core.series` does:

```
import pandas.plotting._core as _gfx  # noqa
Series.plot = base.AccessorProperty(_gfx.SeriesPlotMethods,
                                                              _gfx.SeriesPlotMethods)
Series.hist = _gfx.hist_series
```

This PR moves the import of `plotting._core` to the top of the module and defines `plot` and `hist` inside the `class Series` block.

Initially this caused a bunch of test failures  because `plotting._core` itself imports `Series` (and `plotting._tools`, which also imports `Series`).  #16913 and #16931 removed these circular imports, but mysteriously that didn't solve the problem.

The one that I missed last time around was `pandas.plotting.__init__`, which imports `pandas.plotting._converter`, which imports `Series`.  Changing that to `ABSeries` in addition to the earlier ones finally fixes the problem.

 - [ ] closes #xxxx
 - [ ] tests added / passed
 - [x] passes ``git diff upstream/master -u -- "*.py" | flake8 --diff``
 - [ ] whatsnew entry
