ARROW-7032: [Release] Run the python unit tests in the release verification script
ARROW-7032: [Release] Run the python unit tests in the release verification script

Closes #6170 from kszucs/ARROW-7032 and squashes the following commits:

785a0cddf <Krisztián Szűcs> install pandas
46356f7fc <Krisztián Szűcs> correct ci directory
a97de0637 <Krisztián Szűcs> Run python unit tests during wheel verification

Authored-by: Krisztián Szűcs <6bab32dc957a05a87930e779f272342e159326fa@gmail.com>
Signed-off-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@clear-code.com>

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