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.

--- a/scripts/logformat.py	Sat Jan 25 22:13:43 2020 +0100
+++ b/scripts/logformat.py	Sat Nov 23 22:08:18 2019 +0100
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2
+#!/usr/bin/env python3
 
 from __future__ import print_function