GitHub Actions: fast-fail on black formatting issues (#4268)

* GitHub Actions: fast-fail on black formatting issues

Give fast feedback to contributors https://github.com/psf/black#github-actions

* updating DIRECTORY.md

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>

Author: cclauss

.github/workflows/pre-commit.yml

@@ -14,6 +14,7 @@ jobs:
             ~/.cache/pip
           key: ${{ runner.os }}-pre-commit-${{ hashFiles('.pre-commit-config.yaml') }}
       - uses: actions/setup-python@v2
+      - uses: psf/black@stable
       - name: Install pre-commit
         run: |
           python -m pip install --upgrade pip

DIRECTORY.md

@@ -472,6 +472,7 @@
   * [Runge Kutta](https://github.com/TheAlgorithms/Python/blob/master/maths/runge_kutta.py)
   * [Segmented Sieve](https://github.com/TheAlgorithms/Python/blob/master/maths/segmented_sieve.py)
   * Series
+    * [Arithmetic Mean](https://github.com/TheAlgorithms/Python/blob/master/maths/series/arithmetic_mean.py)
     * [Geometric Mean](https://github.com/TheAlgorithms/Python/blob/master/maths/series/geometric_mean.py)
     * [Geometric Series](https://github.com/TheAlgorithms/Python/blob/master/maths/series/geometric_series.py)
     * [Harmonic Series](https://github.com/TheAlgorithms/Python/blob/master/maths/series/harmonic_series.py)