Affine

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- Addition of new ``eccentricity`` and ``rotation_angle`` properties (28).

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- This is the final 2.0.0 release. The post-release version segment is used
because we accidentally uploaded a 2.0.0 to PyPI before the beta releases
below.

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- Bug fix: restore ``Affine __rmul__`` even though it permits dubious
vector * matrix expressions (27).

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- Breaking change: precision used in properties like ``is_conformal`` is no
longer a global module attribute, but an Affine class or instance attribute
(19, 20).
- Breaking change: ``is_degenerate`` property is now exact and not subject to
a level of precision (23).
- Breaking change: we have reversed our sense of rotation, a positive angle
now rotates a point counter-clockwise about the pivot point (25).
- Bug fix: a bug in matrix-vector multiplication had been reversing the
direction of rotation and is now fixed (25).

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- is_degenerate predicate is precise, not approximate (22).

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- Enable pickling of Affine objects (14).
- Sort out the mixed up shearing parameters (12).