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Harry Terkelsen db8a64f90d

Verify old version of Python has the lib2to3 import available (#170187 )

The `yapf.sh` script checks for specific "known-to-work-with-YAPF"
versions of Python to run YAPF with. On my machine, I have `python3.11`
and `python3.12` (`python3` is a symlink to `python3.12`). However, the
`python3.11` release doesn't have `lib2to3` so the YAPF script fails
with:

```
Checking Python formatting...
To fix, run `et format` or:                                                                                   

git apply <<DONE
  /usr/local/google/home/het/Projects/flutter/engine/src/flutter/tools/gn
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/usr/local/google/home/het/Projects/flutter/engine/src/flutter/third_party/yapf/yapf/__main__.py", line 16, in <module>
    import yapf
  File "/usr/local/google/home/het/Projects/flutter/engine/src/flutter/third_party/yapf/yapf/__init__.py", line 35, in <module>
    from yapf.yapflib import errors
  File "/usr/local/google/home/het/Projects/flutter/engine/src/flutter/third_party/yapf/yapf/yapflib/errors.py", line 16, in <module>
    from lib2to3.pgen2 import tokenize
ModuleNotFoundError: No module named 'lib2to3'

DONE
```

On my Ubuntu machine I cannot install `lib2to3` through normal APT
packages; `apt install python3.11-lib2to3` fails since that package has
been obsoleted. I've been able to work around this by manually editing
the `yapf.sh` script to check if `lib2to3` can be imported before using
that version of Python.

See https://github.com/flutter/flutter/issues/158384

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