Deprecate mpl.py (was Remove mpl.py)

~~`matplotlib/mpl.py` is a file which gets imported via ipython's `%pyplot` interface as module named `mpl`, yet it isn't clear at all what it's role is (it has no docstring).~~

~~Given that in our own code, and in the rest of our python neighborhood, we often do
`import matplotlib as mpl`, it's quite a bit confusing to have something with the `mpl` name within matplotlib not be the `matplotlib` module itself.~~

~~This module was only used in the `pylab` interface, where it was simply imported in one line via `from matplotlib import mpl` and never used again. This PR changes that one line to be `import matplotlib as mpl` and removes `matplotlib/mpl.py` altogether~~

~~The only thing `mpl.py` that doesn't get imported if, in `pylab.py`, we change `from matplotlib import mpl` toand get rid of `mpl.py` altogether, is the `from matplotlib import finance`. I have added a commit here to retain that import, just in case. Howver, I'm not sure it's the right thing to do, because now it preserve a bit of a dichotomy between `import matplotlib as mpl` and  `from pylab import mpl`, where the latter **will** have `mpl.finance`, and the former will not.~~

~~I'm open to removing  4863826 from this PR in order to not squelch this dichotomy, and document such potential backwards incompatibility instead.~~

**UPDATED**: switched to deprecation of mpl.py

`matplotlib/mpl.py` is a file which used to get imported via ipython's
`%pyplot` interface as module named `mpl`, yet it isn't clear at all
what it's role is.

Given that in our own code, and in the rest of our python neighborhood,
we often do `import matplotlib as mpl`, it's quite a bit confusing to
have something with the `mpl` name within matplotlib not be the
`matplotlib` module itself.

This module was only used in `pylab` interface, where it was simply
imported in one line via `from matplotlib import mpl` and never used
again. With the exception of mpl.finance, all of the modules in mpl.py
were readily available simply via "import matplotlib as mpl".

pylab's mpl.finance functionality is retained for backwards
compatability.