Currently,

    import weakref, matplotlib.ft2font
    weakref.ref(matplotlib.ft2font.FT2Font("/usr/share/fonts/TTF/DejaVuSans.ttf"))

aborts Py3 with

    terminate called after throwing an instance of 'char const*'
    Fatal Python error: Aborted

whereas on Py2 a normal `TypeError: cannot create weak reference to
'matplotlib.ft2font.FT2Font' object` is raised.

This is because the following happens:
- Py3 sets the TypeError for failure to create a weakref.
- The FT2Font object gets GC'd as nothing is referring to it (yes,
  creating a weakref to a temporary is a bit silly).
- The FT2Font destructor calls close_file_callback, which calls
  mpl_PyFile_DupClose, which calls PyObject_AsFileDescriptor... which,
  on Py3, calls the fileno() method on the file object.
- PyObject_AsFileDescriptor fails because the originally set TypeError
  has not been cleared, so mpl_PyFile_DupClose likewise fails, causing
  close_file_callback to throw a C++ exception.

Instead, carefully stash and restore the current exception state, both
in mpl_PyFile_DupClose, and likewise below, in mpl_PyFile_CloseFile (the
latter is needed because otherwise PyObject_CallMethod will clear the
exception, cf comment in CPython's Objects/abstract.c:

    /* PyObject_Call() must not be called with an exception set,
    because it may clear it (directly or indirectly) and so the
    caller loses its exception */

Note that if an exception *actually* gets raised by mpl_PyFile_DupClose
or mpl_PyFile_CloseFile, it will overwrite the TypeError.  Strictly
speaking, on Py3, it may be preferrable to chain the exceptions, but
this seems a bit overkill (mostly, I just want to avoid aborting the
whole Python process).