Use Black to format Python files (#14161)

Switch from yapf to Black to better align with the LLVM and broader
Python community. I decided not to go with Pyink as it seems much less
popular and differs in formatting style beyond indentation.

-  Reformat all python files outside of `third_party` with black.
- Update the lint workflow to use black. This only considers files
modified by the PR.
-  Delete old dotfiles.

The command used to reformat all files at once:
```shell
fd -e py --exclude third_party | xargs black
```

To learn more about Back, see: https://black.readthedocs.io/en/stable/
and https://github.com/psf/black.

In the next PR, once the commit SHA of this PR is finalized, I plan to
add this commit to `.git-blame-ignore-revs` to keep the blame history
clean.

Issue: #14135

.github/workflows/lint.yml

@@ -49,7 +49,7 @@ jobs:
           git diff --exit-code
           exit "${EXIT_CODE?}"
 
-  yapf:
+  black:
     runs-on: ubuntu-20.04
     steps:
       - name: Checking out repository
@@ -59,18 +59,18 @@ jobs:
       - name: Fetching Base Branch
         # We have to explicitly fetch the base branch as well
         run: git fetch --no-tags --prune --depth=1 origin "${GITHUB_BASE_REF?}:${GITHUB_BASE_REF?}"
-      - name: Install yapf
+      - name: Install black
         run: |
-          python3 -m pip install yapf==0.30.0
-      - name: Run format_diff.py with yapf
+          python3 -m pip install black==23.3
+      - name: Check if modified files are formatted
         run: |
-          git diff -U0 "${GITHUB_BASE_REF?}" | python3 third_party/format_diff/format_diff.py yapf -i
-          git diff --exit-code
+          git diff "${GITHUB_BASE_REF?}" --name-only -- '*.py' ':!third_party' \
+            | xargs black --check --diff --verbose
       - name: Instructions for fixing the above linting errors
         if: failure()
         run: |
-          printf "You can fix the lint errors above by running\n"
-          printf "  git diff -U0 "${GITHUB_BASE_REF?}" | python3 third_party/format_diff/format_diff.py yapf -i\n"
+          printf "You can fix formatting by running 'black' on the modified python files:\n"
+          printf "  git diff ${GITHUB_BASE_REF?} --name-only -- '*.py' ':!third_party' | xargs black\n"
 
   pytype:
     runs-on: ubuntu-20.04