ARROW-7294: [Python] converted_type_name_from_enum(): Incorrect name for INT_64
ARROW-7294: [Python] converted_type_name_from_enum(): Incorrect name for INT_64

Closes #5938 from gawain-bolton/ARROW-7294_converted_type_name_from_enum_bug and squashes the following commits:

c2741c490 <gawain.bolton> ARROW-7294:  converted_type_name_from_enum(): Incorrect name for INT_64

Authored-by: gawain.bolton <80e8b3fb096aaf569606ffef4aafa80d6cbe4594@cfm.fr>
Signed-off-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@clear-code.com>

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