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-4ti2.patch

@@ -1,17 +1,17 @@
 diff -up src/sage/sandpiles/sandpile.py.orig src/sage/sandpiles/sandpile.py
---- src/sage/sandpiles/sandpile.py.orig	2018-08-03 05:10:09.000000000 -0600
-+++ src/sage/sandpiles/sandpile.py	2018-08-29 08:47:29.728055776 -0600
-@@ -342,9 +342,6 @@ from sage.arith.all import falling_facto
+--- src/sage/sandpiles/sandpile.py.orig	2018-10-17 17:14:17.000000000 -0600
++++ src/sage/sandpiles/sandpile.py	2018-10-25 13:18:34.198894883 -0600
+@@ -344,9 +344,6 @@ from sage.arith.all import falling_facto
  from sage.rings.all import Integer, PolynomialRing, QQ, ZZ
- from sage.symbolic.all import I, pi
+ from sage.symbolic.all import I, pi, SR
  
 -# TODO: remove the following line once 4ti2 functions are removed
--path_to_zsolve = os.path.join(SAGE_LOCAL,'bin','zsolve')
+-path_to_zsolve = os.path.join(SAGE_LOCAL, 'bin', 'zsolve')
 -
- class Sandpile(DiGraph):
-     """
-     Class for Dhar's abelian sandpile model.
-@@ -5197,7 +5194,7 @@ class SandpileDivisor(dict):
+ 
+ 
+ def _sandpile_help(cls, usage, verbose=True):
+@@ -5150,7 +5147,7 @@ class SandpileDivisor(dict):
          sign_file.close()
          # compute
          try: