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

from __future__ import absolute_import

__doc__ = """Same as `echo a >> b`, but ensures a changed mtime of b.
Without this svn will not detect workspace changes."""

import os
import stat
import sys

if sys.version_info[0] >= 3:
    text = os.fsencode(sys.argv[1])
    fname = os.fsencode(sys.argv[2])
else:
    text = sys.argv[1]
    fname = sys.argv[2]

f = open(fname, "ab")
try:
    before = os.fstat(f.fileno())[stat.ST_MTIME]
    f.write(text)
    f.write(b"\n")
finally:
    f.close()
inc = 1
now = os.stat(fname)[stat.ST_MTIME]
while now == before:
    t = now + inc
    inc += 1
    os.utime(fname, (t, t))
    now = os.stat(fname)[stat.ST_MTIME]