Fix type annotations in pandas.plotting._core
Part of #25882

pandas.plotting._core currently has these errors:
```
pandas/plotting/_core.py:958: error: Incompatible types in assignment (expression has type "str", base class "MPLPlot" defined the type as "None")
pandas/plotting/_core.py:1177: error: Incompatible types in assignment (expression has type "str", base class "MPLPlot" defined the type as "None")
pandas/plotting/_core.py:1728: error: "type" has no attribute "_kind"
```