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-eclib.patch

@@ -1,22 +0,0 @@
---- src/sage/libs/eclib/wrap.cpp.orig	2018-05-05 16:21:24.000000000 -0600
-+++ src/sage/libs/eclib/wrap.cpp	2018-08-09 21:57:02.055392569 -0600
-@@ -133,8 +133,8 @@ char* Curvedata_isogeny_class(struct Cur
- 
- 
- int mw_process(struct Curvedata* curve, struct mw* m,
--                      const struct bigint* x, const struct bigint* y,
--                      const struct bigint* z, int sat)
-+                      const bigint* x, const bigint* y,
-+                      const bigint* z, int sat)
- {
-   Point P(*curve, *x, *y, *z);
-   if (!P.isvalid())
-@@ -188,7 +188,7 @@ int mw_rank(struct mw* m)
- }
- 
- /* Returns index and unsat long array, which user must deallocate */
--int mw_saturate(struct mw* m, struct bigint* index, char** unsat,
-+int mw_saturate(struct mw* m, bigint* index, char** unsat,
-                        long sat_bd, int odd_primes_only)
- {
-   vector<long> v;