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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engine/src/flutter/tools/yapf.sh

@@ -24,7 +24,7 @@ set -e
 function follow_links() (
   cd -P "$(dirname -- "$1")"
   file="$PWD/$(basename -- "$1")"
-  while [[ -h "$file" ]]; do
+  while [[ -L "$file" ]]; do
     cd -P "$(dirname -- "$file")"
     file="$(readlink -- "$file")"
     cd -P "$(dirname -- "$file")"
@@ -34,16 +34,36 @@ function follow_links() (
 )
 
 SCRIPT_DIR=$(follow_links "$(dirname -- "${BASH_SOURCE[0]}")")
-SRC_DIR="$(cd "$SCRIPT_DIR/../.."; pwd -P)"
-YAPF_DIR="$(cd "$SRC_DIR/flutter/third_party/yapf"; pwd -P)"
+SRC_DIR="$(
+  cd "$SCRIPT_DIR/../.."
+  pwd -P
+)"
+YAPF_DIR="$(
+  cd "$SRC_DIR/flutter/third_party/yapf"
+  pwd -P
+)"
+
+has_lib2to3_check_script="
+import sys
+version = sys.version_info
+try:
+    __import__('lib2to3')
+    print(f'Python3 version {version.major}.{version.minor} has '
+          f'the lib2to3 import.')
+except ImportError:
+    print(f'Python3 version {version.major}.{version.minor} does not have '
+          f'the lib2to3 import.',
+          file=sys.stderr)
+    sys.exit(1)
+"
 
 # TODO: https://github.com/flutter/flutter/issues/158384
 # Migrate to a supported Python formatter.
-if command -v python3.10 &> /dev/null; then
+if command -v python3.10 &>/dev/null && (python3.10 -c "$has_lib2to3_check_script" || exit 1); then
   PYTHON_EXEC="python3.10"
-elif command -v python3.11 &> /dev/null; then
+elif command -v python3.11 &>/dev/null && (python3.11 -c "$has_lib2to3_check_script" || exit 1); then
   PYTHON_EXEC="python3.11"
-elif command -v python3.12 &> /dev/null; then
+elif command -v python3.12 &>/dev/null && (python3.12 -c "$has_lib2to3_check_script" || exit 1); then
   PYTHON_EXEC="python3.12"
 else
   python3 -c "