Merge branch 'yz/p4-py3'

Update "git p4" to work with Python 3. * yz/p4-py3: ci: use python3 in linux-gcc and osx-gcc and python2 elsewhere git-p4: use python3's input() everywhere git-p4: simplify regex pattern generation for parsing diff-tree git-p4: use dict.items() iteration for python3 compatibility git-p4: use functools.reduce instead of reduce git-p4: fix freezing while waiting for fast-import progress git-p4: use marshal format version 2 when sending to p4 git-p4: open .gitp4-usercache.txt in text mode git-p4: convert path to unicode before processing them git-p4: encode/decode communication with git for python3 git-p4: encode/decode communication with p4 for python3 git-p4: remove string type aliasing git-p4: change the expansion test from basestring to list git-p4: make python2.7 the oldest supported version