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-maxima.patch

@@ -1,6 +1,6 @@
 diff -up src/bin/sage-maxima.lisp.orig src/bin/sage-maxima.lisp
---- src/bin/sage-maxima.lisp.orig	2018-08-03 05:10:08.000000000 -0600
-+++ src/bin/sage-maxima.lisp	2018-08-29 08:45:21.302643791 -0600
+--- src/bin/sage-maxima.lisp.orig	2018-10-17 17:13:34.000000000 -0600
++++ src/bin/sage-maxima.lisp	2018-10-25 13:14:53.465538639 -0600
 @@ -4,3 +4,8 @@
  
  (setf *prompt-prefix* "<sage-display>")
@@ -11,12 +11,12 @@ diff -up src/bin/sage-maxima.lisp.orig src/bin/sage-maxima.lisp
 +  *standard-input* (open "/dev/stdin" :direction :input)
 +  *standard-output* (open "/dev/stdout" :direction :output))
 diff -up src/sage/interfaces/maxima.py.orig src/sage/interfaces/maxima.py
---- src/sage/interfaces/maxima.py.orig	2018-08-03 05:10:08.000000000 -0600
-+++ src/sage/interfaces/maxima.py	2018-08-29 08:47:09.779302448 -0600
+--- src/sage/interfaces/maxima.py.orig	2018-10-17 17:13:51.000000000 -0600
++++ src/sage/interfaces/maxima.py	2018-10-25 13:16:00.326343444 -0600
 @@ -546,7 +546,7 @@ class Maxima(MaximaAbstract, Expect):
          Expect.__init__(self,
                          name = 'maxima',
-                         prompt = '\(\%i[0-9]+\) ',
+                         prompt = r'\(\%i[0-9]+\) ',
 -                        command = 'maxima -p "{0}"'.format(STARTUP),
 +                        command = 'maxima --disable-readline -p "{0}"'.format(STARTUP),
                          env = {'TMPDIR': str(ECL_TMP)},