Matplotlib-venn

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- Fixes issue 24.
- Addresses Debian bug 813782.

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- Fixed issue 17. This would change the previous layout of circles in certain pathological cases.

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- Completely rewritten the region generation logic, presumably fixing all of the problems behind issue 14
(and hopefully not introducing too many new bugs). The new algorithm positions the labels in a different way,
which may look slightly worse than the previous one in some rare cases.
- New kind of IPython-based tests.

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- Better support for weird special cases in Venn3 (i.e. one circle being completely inside another, issue 10).

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- Added support for Python 3.

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- Added the possibility to provide sets (rather than subset sizes) to venn2 and venn3.
Thanks to https://github.com/aebrahim
- Functions won't bail out on sets of size 0 now (the diagrams won't look pretty, though).
Thanks to https://github.com/olgabot
- Venn2/Venn3 objects now provide information about the coordinates and radii of the circles.
- Utility functions added for drawing unweighed diagrams (venn2_unweighted, venn3_unweighted)
- Labels for zero-size sets can be switched off using a method of VennDiagram.
- Some general code refactoring.