[python] Fix exception in request body with enum ref

[wing328]

• Fix exception in request body with enum ref
• add a test
• minor formatting improvement

To close #17171

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cc
@cbornet (2017/09) @tomplus (2018/10) @krjakbrjak (2023/02) @fa0311 (2023/10) @multani (2023/10)

[multani]

Looks good to me, thanks for the additional test 👍