dev-python/matplotlib: bump to 3.1.2.

- Unbundle qhull.
  - Depend on pyobject:3[cairo] for USE="gtk3 cairo".
  - p/arch/arm64/package.use.mask: mask USE=wxwidgets
    dev-python/wxpython does not have arm64 keyword.

Closes: https://bugs.gentoo.org/671796
Closes: https://bugs.gentoo.org/680176
Closes: https://bugs.gentoo.org/665560
Package-Manager: Portage-2.3.79, Repoman-2.3.18
Signed-off-by: Benda Xu <heroxbd@gentoo.org>

dev-python/matplotlib/files/matplotlib-3.1.2-qhull.patch

@@ -0,0 +1,20 @@
+Index: matplotlib-3.1.2/setupext.py
+===================================================================
+--- matplotlib-3.1.2.orig/setupext.py
++++ matplotlib-3.1.2/setupext.py
+@@ -839,14 +839,7 @@ class Qhull(SetupPackage):
+     name = "qhull"
+ 
+     def add_flags(self, ext):
+-        # Qhull doesn't distribute pkg-config info, so we have no way of
+-        # knowing whether a system install is recent enough.  Thus, always use
+-        # the vendored version.
+-        ext.include_dirs.insert(0, 'extern')
+-        ext.sources.extend(sorted(glob.glob('extern/libqhull/*.c')))
+-        if sysconfig.get_config_var('LIBM') == '-lm':
+-            ext.libraries.extend('m')
+-
++        ext.libraries.extend(['qhull'])
+ 
+ class TTConv(SetupPackage):
+     name = "ttconv"