`make_cache_key()` has been with us for a long time, and though it generally worked as expected, there's been room for improvement. This change starts by fixing up the normalization code for invalid characters. Rather than stripping a few select characters (space, tab, newlines), which wasn't sufficient, we now encode the hex values of any invalid characters. According to Django's own modern validation code, these are any characters in the ASCII range of 0-20, or ASCII 127 (DEL). These become literal `'\x00'` through `'\x20'` and `'\x7f'`. The new code uses regexes to do this, which should also be a bit faster. The hash for the key now uses SHA256 instead of MD5, which is less likely to generate collisions. This will of course invalidate any existing keys, but that won't matter because of the final point. We now return Unicode strings instead of byte strings. Turns out we made a mistake when porting this to Python 3. We began returning byte strings when we should have returned native strings. Django expects Unicode strings in the case of Python 3. Behind the scenes, what ended up happening was that a key like `'abc123'` was turning into `b'abc123'` in the cache. This could in theory result in keys that were too big for the cache, when dealing with large cache keys + large data split in `cache_memoize_iter()` (but in practice this hasn't happened). Comprehensive unit tests have been added for `make_cache_key()`.