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.