Mat2 (the cli) will now copy the input file permissions
to the output file.

Due to bubblewrap's pickiness, mat2 can now be run
without a sandbox, even if bubblewrap is installed.

Apparently, abstractstaticmethod is deprecated
since python3.3.

This mounts a new tmpfs on /tmp so any files residing there would be hidden
from the sandbox. Many programs store some files in there that might be useful
to an attacker.  It also drops all capabilities incase it is ever run with
extra capabilities for whatever reason.

On some machines (like mine), `/proc` has to be mounted.  Also, since
sandboxing with bubblewrap is best effort and assumes that an attacker doesn't
have control outside of the file to clean, it's safe to __try__ to enable some
bubblewrap features, and to silently fail otherwise.

This is related to the previous commit

nsids are random identifiers, usually used to ease merging
between documents, and can trivially be used for fingerprinting.

- gentoo and debian with bubblewrap are not allowed to fail anymore
- don't run coverage on debian without bubblewrap

It seems, there is a bug somewhere if the test suite is invoked as
'root', and bubblewrap is available.

Relates #106

As such, hopefully, it's not really used widely anymore. If so, this
note isn't really relevant.

Also, make the note generic, to omit the need to update it "constantly".

Closes #76

This shouldn't make a big difference in the CLI/extension
usage, but might improve the performances of long-running
instances, or people misusing the API.