--chunker-params=fail,4096,rrrEErrrr means:
- cut chunks of 4096b fixed size (last chunk in a file can be less)
- read chunks 0, 1 and 2 successfully
- error at chunk 3 and 4 (simulated OSError(errno.EIO))
- read successfully again for the next 4 chunks
Chunks are counted inside the chunker instance, starting
from 0, always increasing while the same instance is used.
Read chunks as well as failed chunks count up by 1.
also added some .pyi files needed to check the cython code (taken from #5703 and updated).
fixed "syntax error" in key.py.
all mypy complaints not fixed yet.