Update '#!/usr/bin/python' shebangs to '#!/usr/bin/env python'.

Python2 has been considered obsolete since 2020-01-01. Some Linux
distributions are increasingly moving away from installing a
Python2 binary in `/usr/bin/python`, which means that any
Unix shebang lines that refer to it will stop working.

This CL changes (almost) all of the shebang lines found in
chromium/src to use `#!/usr/bin/env python` instead, which is
more portable (it allows you to have your own Python as long as
it is simply somewhere in $PATH). There are a few cases where
I did not update the changes because they were in third_party

The actual command used to generate this CL was:

git grep -l '^#!\s*/usr/bin/python$' -- \*.py | egrep -v 'external/wpt|third_party/(abseil-cpp|android_platform|libxml|closure_compiler|google_input_tools|libxml|mako|opus|protobuf|wpt_tools|zxcvbn-cpp)' | xargs perl -pi -e 's-^#!/usr/bin/python-#!/usr/bin/env python-'

Bug: 1191100
Change-Id: Ifca134a1e24d56b25114c6f70a173d71fec8e6d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2780719
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Owners-Override: Kentaro Hara <haraken@chromium.org>
Owners-Override: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#866002}
GitOrigin-RevId: 4d164bb06345c0aee71d39f1c3affbda327c23e8
2 files changed