Update to sagemath 8.7.

- Drop upstreamed -giac patch.
- Add -sagetex patch to fix a python indentation error.
- Add -rubiks patch to fix compilation of the rubiks library.
- Add -random patch to fix a non-random random bit generator.
- Drop pip3 workaround; the binary is now named just pip again.

sagemath-giac.patch

@@ -1,15 +0,0 @@
-diff -up src/sage/interfaces/giac.py.orig src/sage/interfaces/giac.py
---- src/sage/interfaces/giac.py.orig	2018-12-22 16:37:08.000000000 -0700
-+++ src/sage/interfaces/giac.py	2019-01-15 15:10:47.399759655 -0700
-@@ -617,10 +617,7 @@ If you got giac from the spkg then ``$PR
-             '4\n3'
-             sage: s='g(x):={\nx+1;\nx+2;\n}'
-             sage: giac(s)
--            (x)->{
--            x+1;
--            x+2;
--            }
-+            (x)->[x+1,x+2]
-             sage: giac.g(5)
-             7
-         """