Updated doctests to Python 3.5 and automatically running with Tox.

I've also removed doctests from the obsolete rsa/_versionXXX.py files,
as those files aren't even compatible with Python 3.x anyway.
diff --git a/create_timing_table.py b/create_timing_table.py
index 48cd1ff..1c31b98 100755
--- a/create_timing_table.py
+++ b/create_timing_table.py
@@ -39,5 +39,6 @@
           (bitsize, dur_per_call, iterations, duration))
 
 
-for bitsize in (128, 256, 384, 512, 1024, 2048, 3072, 4096):
-    run_speed_test(bitsize)
+if __name__ == '__main__':
+    for bitsize in (128, 256, 384, 512, 1024, 2048, 3072, 4096):
+        run_speed_test(bitsize)