global: use python3 in shebangs Python 3 is the future. We want Python scripts to be using Python 3 by default. This change updates all `#!/usr/bin/env python` shebangs to use `python3`. Does this mean all scripts use or require Python 3: no. In the test environment, the `PATH` environment variable in tests is updated to guarantee that the Python executable used to run run-tests.py is used. Since test scripts all now use `#!/usr/bin/env python3`, we had to update this code to install a `python3` symlink instead of `python`. It is possible there are some random scripts now executed with the incorrect Python interpreter in some contexts. However, I would argue that this was a pre-existing bug: we should almost always be executing new Python processes using the `sys.executable` from the originating Python script, as `python` or `python3` won't guarantee we'll use the same interpreter. Differential Revision: https://phab.mercurial-scm.org/D9273

#!/usr/bin/env python3
#
# A portable replacement for 'seq'
#
# Usage:
#   seq STOP              [1, STOP] stepping by 1
#   seq START STOP        [START, STOP] stepping by 1
#   seq START STEP STOP   [START, STOP] stepping by STEP

from __future__ import absolute_import, print_function
import os
import sys

try:
    import msvcrt

    msvcrt.setmode(sys.stdin.fileno(), os.O_BINARY)
    msvcrt.setmode(sys.stdout.fileno(), os.O_BINARY)
    msvcrt.setmode(sys.stderr.fileno(), os.O_BINARY)
except ImportError:
    pass

if sys.version_info[0] >= 3:
    xrange = range

start = 1
if len(sys.argv) > 2:
    start = int(sys.argv[1])

step = 1
if len(sys.argv) > 3:
    step = int(sys.argv[2])

stop = int(sys.argv[-1]) + 1

for i in xrange(start, stop, step):
    print(i)