[devtools] Install yapf in virtualenv

I figured out that PYTHONPATH was an issue.

TODO: make separate wedges for yapf and MyPy deps.  I guess the wedges
are virtualenvs with binary artifacts.

build/deps.sh

@@ -280,8 +280,19 @@ install-py3-libs-in-venv() {
   time python3 -m pip install -r $mypy_dir/test-requirements.txt
 
   # pexpect: for spec/stateful/*.py
-  # yapf: all devs should on the same version
-  python3 -m pip install pexpect yapf
+  python3 -m pip install pexpect
+
+  # TODO:
+  # - Do something like this 'pip download' in build/deps.sh fetch
+  # - Then create a WEDGE which installs it
+  #   - However note that this is NOT source code; there is binary code, e.g.
+  #   in lxml-*.whl
+  if false; then
+    local pip_dir=_tmp/pip
+    mkdir -p $pip_dir
+    python3 -m pip download -d $pip_dir -r $mypy_dir/test-requirements.txt
+    python3 -m pip download -d $pip_dir pexpect
+  fi
 }
 
 install-py3-libs() {

devtools/format.sh

@@ -8,6 +8,10 @@
 REPO_ROOT=$(cd "$(dirname $0)/.."; pwd)
 
 . build/dev-shell.sh  # python3 in $PATH
+
+# Hack to prevent interference.  TODO: Make a seperate wedge for yapf.
+unset PYTHONPATH
+
 . build/common.sh  # $CLANG_DIR
 
 set -o nounset
@@ -19,13 +23,35 @@ shopt -s strict:all 2>/dev/null || true  # dogfood for OSH
 # Python
 #
 
+readonly YAPF_VENV='_tmp/yapf-venv'
+
+install-yapf() {
+  local venv=$YAPF_VENV
+
+  rm -r -f -v $venv
+
+  python3 -m venv $venv
+
+  . $venv/bin/activate
+
+  # 0.40.1 is the 2023-06-20 release
+  #
+  # Pin the version so formatting is stable!
+
+  python3 -m pip install 'yapf == 0.40.1'
+
+  yapf-version
+}
+
 yapf-version() {
+  . $YAPF_VENV/bin/activate
   python3 -m yapf --version
 }
 
 # For now, run yapf on specific files.  TODO: could query git for the files
 # that are are different from master branch, and run it on those.
 yapf-files() {
+  . $YAPF_VENV/bin/activate
   python3 -m yapf -i "$@"
 }