Relates 0a2a398c

We didn't take the whitelist into account while
removing dangling files from [Content_types].xml

- Replace some class members by instance members
- Don't thread the cleaning process anymore for now

This should fix the intermittent failures, thanks
to @pollo for the tip

MS Office XML rsid is a "unique identifier used to track the editing session
when the physical character representing this section mark was last formatted."

See the following links for details:
- https://msdn.microsoft.com/en-us/library/office/documentformat.openxml.wordprocessing.previoussectionproperties.rsidrpr.aspx
- https://blogs.msdn.microsoft.com/brian_jones/2006/12/11/whats-up-with-all-those-rsids/.

In XML, the order of the attributes shouldn't be meaningful,
however, MS Office sorts attributes for a given XML tag
differently than LibreOffice.

Yoann Lamouroux authored 6 years ago and jvoisin committed 6 years ago

`/usr/bin/python3` -> `/usr/bin/env python3`

It's always better to trust the environment defined path to bin/python, as
virtualenv become the way to go.

Closes #60

Note: this changeset also ensures that clean.cleaned.docx is removed
up after the pytest is over.

while mat2 has both a thread model (a thread pool that strips metadata
in parallel) and a threat model (a list of malicious adversaries and
their capabilities that we are trying to defeat), i think this
paragraph is talking about the latter.