### What changes were proposed in this pull request?
Replace `__metaclass__` fields with `metaclass` keyword in the class statements.
### Why are the changes needed?
`__metaclass__` is no longer supported in Python 3. This means, for example, that types are no longer handled as singletons.
```
>>> from pyspark.sql.types import BooleanType
>>> BooleanType() is BooleanType()
False
```
and classes, which suppose to be abstract, are not
```
>>> import inspect
>>> from pyspark.ml import Estimator
>>> inspect.isabstract(Estimator)
False
```
### Does this PR introduce _any_ user-facing change?
Yes (classes which were no longer abstract or singleton in Python 3, are now), though visible changes should be consider a bug-fix.
### How was this patch tested?
Existing tests.
Closes#29664 from zero323/SPARK-32138-FOLLOW-UP-METACLASS.
Authored-by: zero323 <mszymkiewicz@gmail.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
c918909c1a