tests/float: Convert "sys.exit()" to "raise SystemExit".

The latter is shorter and simpler because it doesn't require importing the
sys module.

tests/float/cmath_fun.py

@@ -4,8 +4,7 @@ try:
     from cmath import *
 except ImportError:
     print("SKIP")
-    import sys
-    sys.exit()
+    raise SystemExit
 
 # make sure these constants exist in cmath
 print("%.5g" % e)