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

ci/lib.sh

@@ -162,6 +162,9 @@ linux-clang|linux-gcc)
 	if [ "$jobname" = linux-gcc ]
 	then
 		export CC=gcc-8
+		MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=$(which python3)"
+	else
+		MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=$(which python2)"
 	fi
 
 	export GIT_TEST_HTTPD=true
@@ -182,6 +185,9 @@ osx-clang|osx-gcc)
 	if [ "$jobname" = osx-gcc ]
 	then
 		export CC=gcc-9
+		MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=$(which python3)"
+	else
+		MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=$(which python2)"
 	fi
 
 	# t9810 occasionally fails on Travis CI OS X