Milestone 2.0.0 planning and call for testers

[ewl+rdiffbackup]

Hi,

I've just updated the milestone planning for 2.0.0, adding goals and 
tagging issues. There is also a tentative date of end of the year.

The goals are (and they are subject to discussion and changes):

1. works under Python 3 (Python 2 support dropped)
2. no regression (works at least as good as rdiff-backup 1.2.8 under 
Linux, Windows _64 bits_ and MacOS/X)
3. automated pipeline for easy and stable testing and packaging

All issues are aligned with the goals and have an owner, with the 
exception of issue [#82], which requires an owner to develop and test on 
MacOS/X else we'll have to remove MacOS from our milestone goals and 
supported environments.

Overall, and especially under Windows, we need testers to make sure we 
avoided regressions. Please test what you're doing both under 1.2.8 and 
1.3.6 or better from master, and create an issue with both logs.

KR, Eric

[1] https://github.com/rdiff-backup/rdiff-backup/milestone/1
[#82] https://github.com/rdiff-backup/rdiff-backup/issues/82