BLD: fix typo in black task for Github Actions

Author: qwhelan

.github/workflows/pythonpackage.yml

@@ -31,7 +31,7 @@ jobs:
         # exit-zero treats all errors as warnings. The GitHub editor is 127 chars wide
         flake8 . --count --statistics
     - name: Check formatting with black
-      if: matrix.python.version >= 3.6
+      if: matrix.python-version >= 3.6
       run: |
         pip install black
         black . --check --exclude "(build/|dist/|\.git/|\.mypy_cache/|\.tox/|\.venv/\.asv/|env|\.eggs)"