scripts: use plain import of contributor_data

Relative import failed with:

Traceback (most recent call last):
File "scripts/update-copyrights.py", line 45, in <module>
from . import contributor_data
ImportError: attempted relative import with no known parent package

Fixed by backing out changeset 2786730e56e0. The pytype problem can be solved
differently.

py3: switch to use Python 3 interpreter, temporarily leaving many things very broken until they have been migrated/fixed in a reviewable way

Bump Mercurial minimum version to 5.2 - the first version that claim stable py3
support.

scripts: use explicit relative imports of contributor_data

Avoid any ambiguity ... and avoid pytype failing to find
scripts/contributor_data.py when scripts isn't in sys.path.

scripts/shortlog: new script

Useful for release announcements, shortlog.py presents a list of committers
corresponding to the specified revision set, along with their count of
commits in that set.

Example usage:
scripts/shortlog.py "only('.', branch('stable') & tagged() & public() & not '.')"