Check nbgrader version + PEP 8 + mypy config

mypy.ini

+[mypy]
+warn_return_any = True
+warn_unused_configs = True
+disallow_untyped_defs = True
+disallow_untyped_calls = True
+disallow_incomplete_defs = True
+check_untyped_defs = True
+no_implicit_optional = True
+scripts_are_modules = True

test_python_packages.py

-def test_python_version():
+def test_python_version() -> None:
     import sys
     assert sys.version_info.major == 3
     assert sys.version_info.minor == 8
 
-def test_tensorflow():
-    import tensorflow
+
+def test_tensorflow() -> None:
+    import tensorflow  # type: ignore
+    tensorflow
+
+
+def test_nbgrader() -> None:
+    import nbgrader    # type: ignore
+    assert nbgrader.version_info == (0, 7, 0, 'dev')