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-random.patch

@@ -0,0 +1,32 @@
+diff -up src/sage/misc/binary_tree.pyx.orig src/sage/misc/binary_tree.pyx
+--- src/sage/misc/binary_tree.pyx.orig	2019-03-23 16:20:41.000000000 -0600
++++ src/sage/misc/binary_tree.pyx	2019-04-09 13:55:17.152716497 -0600
+@@ -13,6 +13,8 @@ from cysignals.memory cimport sig_malloc
+ 
+ from cpython.ref cimport PyObject, Py_INCREF, Py_XDECREF
+ 
++from sage.misc.prandom import randint
++
+ cdef binary_tree_node *BinaryTreeNode(int key, object value):
+     cdef binary_tree_node *t
+     t = <binary_tree_node *>sig_malloc(sizeof(binary_tree_node))
+@@ -122,10 +124,7 @@ cdef binary_tree_node *binary_tree_right
+ cdef binary_tree_node *binary_tree_head_excise(binary_tree_node *self):
+     cdef binary_tree_node *cur
+     cdef int right
+-    # We have a pointer we're about to free.  Chances are, we'll never
+-    # see this pointer again.  Thus, its least significant bit is
+-    # "random" enough to resist bias.
+-    right = (<int>self)&1
++    right = randint(0, 1)
+     if self.right == NULL:
+         return self.left
+     if self.left == NULL:
+@@ -502,7 +501,6 @@ class Test:
+ 
+             sage: sage.misc.binary_tree.Test().random()
+         """
+-        from sage.misc.prandom import randint
+         t = BinaryTree()
+         for i in xrange(cycles):
+             r = randint(0,8)