chore(build.yml): bump python version, remove scipy as bnb extra

.github/workflows/build.yml

@@ -14,18 +14,17 @@ jobs:
     steps:
     - uses: actions/checkout@v2
 
-    - name: Set up Python 3.8
+    - name: Set up Python 3.9
       uses: actions/setup-python@v2
       with:
-        python-version: 3.8
+        python-version: 3.9
 
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
         pip install -r requirements.txt
         # Install extras
-        # [bnb] (TODO: Remove `scipy` once `bnb` adds it as hard dep)
-        pip install bitsandbytes scipy
+        pip install bitsandbytes
         # [dev]
         pip install black hypothesis isort flake8 pre-commit pytest pytest-cov