Mac binary installer for Python 3 delayed

It turns out I won't be able to make a Mac binary installer for
matplotlib anytime soon. I use bdist_mpkg to make the Mac binary
installers for MacOS X, and the current release of bdist_mpkg (4.4) is
not compatible with Python 3. I don't know how to make binary installers
without bdist_mpkg, though I'm sure it can be done.

Also note that there are no numpy binary installers for MacOS X python
3. This *may* be due to the same issue.

Fortunately, it is very easy to install matplotlib from source (much
easier than it used to be, thanks to patching setupext.py for darwin).

-- Russell