Kwplot

Fixed
* Fixed potential crash in autompl on some systems with conflicting PyQt5 and
python-opencv libraries. This involves falling back to agg if there *might*
be a conflict. The environ KWPLOT_UNSAFE=1 can be set to use the old behavior

Added
* Added "force" kwarg to autompl, which will change the backend and prevent
future non-force autompl calls from changing the backend.

* BackendContext - a context manager for matplotlib backends. Primary purpose
is to allow the user to ensure that the agg backend is used in certain
places.

* `mpl_draw.draw_points` - draws 2d points

Fixed
* Bugs in `plot_matrix`

Changes
* Moved implementation of non-matplotlib drawing function to kwimage. This lets
kwimage be independent of kwplot. These functions are still exposed in kwplot
for backwards compatibility. This may change in the future.

Changes
* Starting a changelog
* Tweaking build scripts and CI configurations.
* Minor improvements to `multi_plot`
* `Color` no longer directly depends on `matplotlib`, might be ported to `kwimage` in the future.