Use flake8 directly instead of through pytest-flake8

pytest-flake8 is failing due to tholo/pytest-flake8#88.

Author: lpsinger

.github/workflows/build-and-test.yml

@@ -23,12 +23,15 @@ jobs:
         python-version: ${{ matrix.python-version }}
     - name: Install dependencies
       run: |
-        pip install pytest-flake8 pytest-cov
+        pip install flake8 pytest-cov
+    - name: Lint
+      run: |
+        flake8 .
     - name: Build
       run: |
         pip install -e .
     - name: Test
       run: |
-        pytest --flake8 --cov --cov-report=xml
+        pytest --cov --cov-report=xml
     - name: Upload coverage to codecov
       uses: codecov/codecov-action@v3