ENH: .astype() with dictionary while preserving type(self)

[sambo55]

Is your feature request related to a problem?

[this should provide a description of what the problem is, e.g. "I wish I could use pandas to do [...]"]

I have a custom DataFrame class with

class CustomFrame(DataFrame):
@property
    def _constructor(self):
        return CustomFrame

which allows me to use normal pandas methods and preserves my custom DataFrame object.

When using df.astype({'col':'type'}) the type returned is always a DataFrame instead of preserving the self._constructor and returning a CustomFrame

When using df.assign(col = col.astype('type')) the type returned is CustomFrame

Describe the solution you'd like

If we could change

result = result.__finalize__(self, method="astype")

to

result = self._constructor(results).__finalize__(self, method="astype")

then the constructor call would ensure the correct type is returned which is what is used in the col.astype('type') case
self._constructor(new_data).__finalize__(self, method="astype")

API breaking implications

I don't think this would break anything, the resulting objects in both cases are the same except for the type.

Describe alternatives you've considered

I could just use the assign pattern as a workaround but it would be nice if we could also use the dictionary notation

Additional context

# Your code here, if applicable

[phofl]

Duplicate of #40810