- The `KjException._to_python()` function neglected to check if the wrapper was
set when attempting to convert to `AttributeError`, leading to exceptions while
raising an exception.
- The syntax `raise A, B, C` hasn't existed since Python 3. The only reason it
works is because Cython supports it. Lets get rid of it.
- There was an attempt to convert a certain kind of `KjException` to an
`AttributeError`. However, the original exception remains in the context when
the new exception is raised. This is confusing. We get rid of the original
exception by doing `raise e._to_python() from None`.
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