matplotlib/matplotlib — pull request #1565 by ivanov (2012-12-05 21:33:57+00:00)
Title: new MatplotlibDeprecationWarning class

In light of the fact that Python builtin DeprecationWarnings are ignored
by default as of Python 2.7 (see link below), this class was put in to
allow for the signaling of deprecation, but via UserWarnings which are
not ignored by default.

http://docs.python.org/dev/whatsnew/2.7.html#the-future-for-python-2-x

Prior to this commit:

```
In [1]: %pylab

Welcome to pylab, a matplotlib-based Python environment [backend: agg].
For more information, type 'help(pylab)'.

In [2]: mlab.liaupunov([1,2], np.diff)
Out[2]: 0.0
```

After this commit:
```
In [1]: %pylab

Welcome to pylab, a matplotlib-based Python environment [backend: agg].
For more information, type 'help(pylab)'.

In [2]: mlab.liaupunov([1,2], np.diff)
/home/pi/.local/lib/python2.7/site-packages/matplotlib/mlab.py:1212:
MatplotlibDeprecationWarning: This does not belong in matplotlib and
will be removed
mDeprecation) # 2009/06/13
Out[2]: 0.0
```

this was motivated by the discussion over in #1535