Increase test coverage for fields.py

tests/conftest.py

@@ -0,0 +1,13 @@
+import pytest
+from openpyxl.workbook import Workbook
+from openpyxl.worksheet.worksheet import Worksheet
+
+
+@pytest.fixture
+def workbook() -> Workbook:
+    return Workbook()
+
+
+@pytest.fixture
+def worksheet(workbook: Workbook) -> Worksheet:
+    return Worksheet(workbook)