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-atlas.patch

@@ -1,39 +0,0 @@
-diff -up src/module_list.py.orig src/module_list.py
---- src/module_list.py.orig	2018-05-16 14:13:22.021406801 -0600
-+++ src/module_list.py	2018-05-16 14:15:57.654208949 -0600
-@@ -12,7 +12,7 @@ SAGE_INC = '/usr/include'
- import pkgconfig
- 
- # CBLAS can be one of multiple implementations
--cblas_pc = pkgconfig.parse('cblas')
-+cblas_pc = pkgconfig.parse('atlas')
- cblas_libs = cblas_pc['libraries']
- cblas_library_dirs = cblas_pc['library_dirs']
- cblas_include_dirs = cblas_pc['include_dirs']
-@@ -21,11 +21,9 @@ cblas_include_dirs = cblas_pc['include_d
- if os.path.exists('/usr/lib/libblas.dll.a'):
-     cblas_libs = ['gslcblas']
- 
--# LAPACK can be one of multiple implementations
--lapack_pc = pkgconfig.parse('lapack')
--lapack_libs = lapack_pc['libraries']
--lapack_library_dirs = lapack_pc['library_dirs']
--lapack_include_dirs = lapack_pc['include_dirs']
-+lapack_libs = [ 'lapack' ]
-+lapack_library_dirs = []
-+lapack_include_dirs = [ SAGE_INC + '/lapack' ]
- 
- # GD image library
- gd_pc = pkgconfig.parse('gdlib')
-diff -up src/sage/misc/cython.py.orig src/sage/misc/cython.py
---- src/sage/misc/cython.py.orig	2018-05-16 13:16:29.207708998 -0600
-+++ src/sage/misc/cython.py	2018-05-16 14:15:57.656208921 -0600
-@@ -40,7 +40,7 @@ from sage.misc.sage_ostools import resto
- 
- 
- # CBLAS can be one of multiple implementations
--cblas_pc = pkgconfig.parse('cblas')
-+cblas_pc = pkgconfig.parse('atlas')
- cblas_libs = list(cblas_pc['libraries'])
- cblas_library_dirs = list(cblas_pc['library_dirs'])
- cblas_include_dirs = list(cblas_pc['include_dirs'])