fix .astype("string[pyarrow]")

[burnpanck]

• [x] Closes #101
• [x] Executed pre-commit run --all-files with no errors
• [x] The change is fully covered by automated unit tests
• [x] Added an entry to the CHANGES file

Not relevant:

• Documented in docs/ as appropriate

[andrewgsavage]

bors r+

[bors[bot]]

Merge conflict.

[andrewgsavage]

thanks for the pr, found how to run the tests, and used this in #110