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

# Filters traceback lines from stdin.

from __future__ import absolute_import, print_function

import io
import sys

if sys.version_info[0] >= 3:
    # Prevent \r from being inserted on Windows.
    sys.stdout = io.TextIOWrapper(
        sys.stdout.buffer,
        sys.stdout.encoding,
        sys.stdout.errors,
        newline="\n",
        line_buffering=sys.stdout.line_buffering,
    )

state = 'none'

for line in sys.stdin:
    if state == 'none':
        if line.startswith('Traceback '):
            state = 'tb'

    elif state == 'tb':
        if line.startswith('  File '):
            state = 'file'
            continue

        elif not line.startswith(' '):
            state = 'none'

    elif state == 'file':
        # Ignore lines after "  File "
        state = 'tb'
        continue

    print(line, end='')