New external dependency ContourPy used for quad contour calculations

Previously Matplotlib shipped its own C++ code for calculating the contours of quad grids. Now the external library ContourPy is used instead. There is a choice of four algorithms to use, controlled by the algorithm keyword argument to the functions contour and contourf. The default behaviour is to use algorithm='mpl2014' which is the same algorithm that Matplotlib has been using since 2014.

See the ContourPy documentation for further details of the different algorithms.

Note

Contour lines and polygons produced by algorithm='mpl2014' will be the same as those produced before this change to within floating-point tolerance. The exception is for duplicate points, i.e. contours containing adjacent (x, y) points that are identical; previously the duplicate points were removed, now they are kept. Contours affected by this will produce the same visual output, but there will be a greater number of points in the contours.

The locations of contour labels obtained by using clabel may also be different.