Fix type annotations in pandas.core.ops
Part of #25882
- [X] tests passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Corrects these errors in `pandas.core.ops`:
```
pandas/core/ops.py:632: error: Unsupported target for indexed assignment
pandas/core/ops.py:633: error: Value of type "object" is not indexable
pandas/core/ops.py:635: error: "object" has no attribute "copy"
pandas/core/ops.py:636: error: Unsupported target for indexed assignment
pandas/core/ops.py:637: error: Unsupported target for indexed assignment
```
