[3.6] Fix small typos in introduction and datastructures of tutorial (GH-272) by Mariatta · Pull Request #297 · python/cpython

I got a test system and fixed some minor issues:

1. C char is unsigned by default. Comparisons between char and integer
   fail for negative values. AFAIK this affects
   only PyFrameObject.f_executing

2. Stack usage in debug builds without optimization (CFLAGS=-O0).
   Worst case (compiler clang) is about 1560 bytes per Python recursion.
   On AMD64 we need only about 744 bytes. This causes various failures
   in test_pickle.TestPickledTasklets.
   Fix: double the maximum stack size, if compiled without function
   inlining.