Update to sagemath 8.3 (bz 1612867).

Also:
- Drop -lrslib, -gap-hap, and -flask patches.
- Drop obsolete scriplets to fix F24 to F25 upgrade (bz 1594429 and 1618934).
- Drop obsolete mktexlsr invocations.
- Fix more Singular paths.
- Fix still more uses of /usr/bin/env.
- Drop disallow/dissallow fixup for cython; now fixed in cython itself.

sagemath-readonly.patch

@@ -1,6 +1,7 @@
---- src/sage/all.py.orig	2018-05-05 16:21:22.000000000 -0600
-+++ src/sage/all.py	2018-05-16 13:25:28.300415250 -0600
-@@ -290,11 +290,11 @@ def _write_started_file():
+diff -up src/sage/all.py.orig src/sage/all.py
+--- src/sage/all.py.orig	2018-08-03 05:10:08.000000000 -0600
++++ src/sage/all.py	2018-08-29 08:45:06.117831555 -0600
+@@ -284,11 +284,11 @@ def _write_started_file():
  
      Check that the file exists when Sage is running::