we now just treat that one .borg_part file we might have inside
checkpoint archives as a normal file.
people can recognize via the file name it is a partial file.
nobody cares for statistics of checkpoint files and the final
archive now does not contain any partial files any more, thus
no needs to maintain statistics about count and size of part
files.
implemented by introducing one level of indirection, the limit is now
very high, so it is not practically relevant any more.
we always use the indirection (storing the metadata stream chunk ids list not
directly into the archive item, but into some repo objects referenced by the new
ArchiveItem.item_ptrs list).
thus, the code behaves the same for all archive sizes.
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.
Thomas Waldmann