build: use python3 to lint

[nornagon]

Description of Change

python no longer exists on macOS Monterey.

Notes: none

[miniak]

python3 ./script/check-relative-doc-links.py

Traceback (most recent call last):
  File "./script/check-relative-doc-links.py", line 130, in <module>
    sys.exit(main())
  File "./script/check-relative-doc-links.py", line 30, in main
    totalBrokenLinks += getBrokenLinks(path)
  File "./script/check-relative-doc-links.py", line 81, in getBrokenLinks
    newLines = newFile.readlines()
  File "/usr/lib/python3.6/encodings/ascii.py", line 26, in decode
    return codecs.ascii_decode(input, self.errors)[0]
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 1927: ordinal not in range(128)

[dsanders11]

I know my stamp has no effect here, but throwing a request changes on this.

I think if we should be more deliberate if we're going to make this change and choose to cut over to Python 3 entirely, rather than doing a change like this. Especially since a change like this affects pre-commit hooks and the likes.

• Our documentation lists Python 2 as a prerequisite
• Windows CI scripts currently install Python 2
• build-tools uses Python 2 for Windows
• Not all OSs we support for building have a python3 executable
  • As far as I can tell, macOS Mojave (10.14) and earlier don't come with Python 3, and the build instructions list support for >= 10.11.6
  • If you use the official Python 3 installer on Windows, you won't get a python3 executable, that's not a naming convention they implement there. However, python3 is a stub on Windows 10 which will take you to the Windows Store to install it via the store - yea, it's a headache

I'm not opposed to switching to officially switching over to Python 3 if we determine we can do so. However, this change feels like it will potentially cause problems on other OSs just to fix macOS Monterey.

[nornagon]

Python 2 has been officially unsupported since Jan 1 2020. I'm not interested in spending any effort supporting it.

However, we should (a) make sure our scripts actually work in Python 3 and (b) continue to function on Windows, those are important.

[nornagon]

I've removed some outdated Python 2 information from our docs, and also dropped some no-longer-current information about required macOS and SDK versions, as those now shift regularly as we track Chromium.

[jkleinsc]

FWIW... in working on the latest chromium roll, I came across this change to depot_tools:
3525960: Explicitly run everything with python3 | https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3525960.

[nornagon]

Alright, given that change upstream, I'm going to merge this. If depot_tools expects a python3 to exist everywhere, then so can we.

[release-clerk]

No Release Notes

[trop]

I was unable to backport this PR to "15-x-y" cleanly;
you will need to perform this backport manually.

[trop]

I have automatically backported this PR to "16-x-y", please check out #33716

[trop]

I have automatically backported this PR to "17-x-y", please check out #33717

[trop]

I have automatically backported this PR to "19-x-y", please check out #33718

[trop]

I have automatically backported this PR to "18-x-y", please check out #33719

[nornagon]

Followup: #33720