Update to sagemath 8.4.

Also:
- Build for python 3 instead of python 2 due to upcoming python 2 removal.
- Add -python3 and -escape patches to fix problems with python 3.
- Drop -nofstring patch, only needed for python 2.
- Drop upstreamed -eclib patch.
- Switch from atlas to openblas and rename -atlas patch to -openblas.
- Add -buildroot patch and only build cython interfaces once.

sagemath-buildroot.patch

@@ -0,0 +1,12 @@
+--- src/setup.py.orig	2018-10-16 09:07:30.086993782 -0600
++++ src/setup.py	2018-10-16 09:10:20.586710425 -0600
+@@ -387,6 +387,9 @@ class sage_build_cython(Command):
+         with open(self._version_file, 'w') as f:
+             f.write(self._version_stamp)
+ 
++        # Remove buildroot paths from the generated files
++        subprocess.run(["find", "build/cythonized/sage", "-type", "f", "-exec", "sed", "-i", "s|@@BUILDROOT@@||g", "{}", "+"], check=True)
++
+         # Finally, copy relevant cythonized files from build/cythonized
+         # tree into the build-lib tree
+         for (dst_dir, src_files) in self.get_cythonized_package_files():