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-nauty.patch

@@ -1,12 +1,12 @@
 diff -up src/sage/graphs/graph_generators.py.orig src/sage/graphs/graph_generators.py
---- src/sage/graphs/graph_generators.py.orig	2018-05-05 16:21:24.000000000 -0600
-+++ src/sage/graphs/graph_generators.py	2018-05-16 13:35:59.299778933 -0600
-@@ -886,7 +886,7 @@ class GraphGenerators():
-             >A geng -d0D3 n=4 e=0-6
-         """
-         import subprocess
+--- src/sage/graphs/graph_generators.py.orig	2018-08-29 08:52:26.099392035 -0600
++++ src/sage/graphs/graph_generators.py	2018-08-29 08:56:44.782205619 -0600
+@@ -906,7 +906,7 @@ class GraphGenerators():
+         else:
+             enc_kwargs = {'encoding': 'latin-1'}
+ 
 -        sp = subprocess.Popen("geng {0}".format(options), shell=True,
 +        sp = subprocess.Popen("nauty-geng {0}".format(options), shell=True,
                                stdin=subprocess.PIPE, stdout=subprocess.PIPE,
-                               stderr=subprocess.PIPE, close_fds=True)
-         if debug:
+                               stderr=subprocess.PIPE, close_fds=True,
+                               **enc_kwargs)