Update matplotlib to 3.1.2

[pyup-bot]

This PR updates matplotlib from 1.4.0 to 3.1.2.

Changelog

3.1.1

The first bug-fix release of the 3.1 series

* Locator.nonsingular return order API change

Lots of backports of various bug fixes.

2.1.0

This is the second minor release in the Matplotlib 2.x series and the first
release with major new features since 1.5.

This release contains approximately 2 years worth of work by 275 contributors
across over 950 pull requests.  Highlights from this release include:

- support for string categorical values
- export of animations to interactive javascript widgets
- major overhaul of polar plots
- reproducible output for ps/eps, pdf, and svg backends
- performance improvements in drawing lines and images
- GUIs show a busy cursor while rendering the plot


along with many other enhancements and bug fixes.

2.0.0

Major release of Matplotlib

This release completely overhauls the default style of the plots.

Highlights include:
- 'viridis' is default color map instead of jet.
- Modernized the default color cycle.
- Many more functions respect the color cycle.
- Line dash patterns scale with linewidth.
- Change default font to DejaVu, now supports most Western
alphabets (including Greek, Cyrillic and Latin with diacritics),
math symbols and emoji out of the box.
- Faster text rendering.
- Improved auto-limits.
- Ticks out and only on the right and bottom spines by default.
- Improved auto-ticking, particularly for log scales and dates.
- Improved image support (`imshow` respects scales and eliminated a
class of artifacts).

2.0.0rc2

This is the second and final planned release candidate for mpl v2.0

This release includes:
- Bug fixes and documentation changes
- Expanded API on plot_surface and plot_wireframe
- Pin font size at text creation time
- Suppress fc-cache warning unless it takes more than 5s

2.0.0rc1

This is the first release candidate for mpl v2.0

This release includes:
- A re-implementation of the way margins are handled during auto
scaling to allow artists to 'stick' to an edge of the Axes
- Improvements to the ticking with log and symlog scales
- Deprecation of the finance module.  This will be spun off into a stand-alone package
- Deprecation of the 'hold' machinery 
- Bumped the minimum numpy version to 1.7
- Standardization of hatch width and appearance across backends
- Made threshold for triggering 'offset' in `ScalarFormatter` configurable
and default to 4 (plotting against years should just work now)
- Default encoding for mp4 is now h264
- `fill_between` and `fill_betweenx` now use the color cycle
- Default alignment of bars changed from 'edge' to 'center'
- Bug and documentation fixes

2.0.0b4

Fourth and final beta release

2.0.0b3

Third beta for v2.0.0 release

This tag includes several critical bug fixes and updates the dash patterns.

1.5.3

This release contains a few critical bug fixes:
- eliminate fatal exceptions with Qt5.7
- memory leak in the contour code
- keyboard interaction bug with nbagg
- automatic integration with the ipython event loop (if running) which
fixes 'naive' integration for IPython 5+

Links

• PyPI: https://pypi.org/project/matplotlib
• Changelog: https://pyup.io/changelogs/matplotlib/
• Homepage: https://matplotlib.org

[pyup-bot]

Closing this in favor of #421