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-cremona.patch

@@ -1,12 +1,12 @@
 diff -up src/sage/databases/cremona.py.orig src/sage/databases/cremona.py
---- src/sage/databases/cremona.py.orig	2018-08-03 05:10:08.000000000 -0600
-+++ src/sage/databases/cremona.py	2018-08-29 08:48:43.090148635 -0600
+--- src/sage/databases/cremona.py.orig	2018-10-17 17:13:36.000000000 -0600
++++ src/sage/databases/cremona.py	2018-10-25 13:24:02.644939224 -0600
 @@ -53,7 +53,6 @@ from sage.misc.prandom import randint
  
  import sage.schemes.elliptic_curves.constructor as elliptic
  from .sql_db import SQLDatabase, verify_column
 -from sage.misc.package import is_package_installed
- from sage.env import SAGE_SHARE
+ from sage.env import CREMONA_MINI_DATA_DIR, CREMONA_LARGE_DATA_DIR
  from sage.misc.all import walltime
  
 @@ -825,14 +824,6 @@ class MiniCremonaDatabase(SQLDatabase):