MyPy cleanup and absolute imports in pandas.core.dtypes.common
Starting the conversation towards #14468

There were a couple misplaced annotations below docstrings that were causing mypy to choke when running:

```bash
mypy pandas --ignore-missing-imports
```

A starred import in pandas.core.dtypes.common was responsible for a lot of complaints from mypy. These were being intentionally suppressed by flake8 but I figure it makes sense to clean up and convert into absolute imports