ARROW-6392: [FlightRPC][Python] check type of list_flights result

[ghost]

No description provided.

[ghost]

Travis build: https://travis-ci.com/lihalite/arrow/builds/125290425

[codecov-io]

Codecov Report

Merging #5242 into master will increase coverage by 1.57%.
The diff coverage is 73.91%.

@@            Coverage Diff             @@
##           master    #5242      +/-   ##
==========================================
+ Coverage   87.63%    89.2%   +1.57%     
==========================================
  Files        1033      750     -283     
  Lines      148630   107773   -40857     
  Branches     1437        0    -1437     
==========================================
- Hits       130249    96142   -34107     
+ Misses      18019    11631    -6388     
+ Partials      362        0     -362

Impacted Files	Coverage Δ
python/pyarrow/_flight.pyx	54.65% <0%> (+1.23%)	⬆️
cpp/src/arrow/python/flight.cc	96.77% <100%> (+3.91%)	⬆️
python/pyarrow/tests/test_flight.py	70.92% <77.77%> (+0.25%)	⬆️
js/src/util/fn.ts
go/arrow/memory/memory_avx2_amd64.go
rust/datafusion/src/execution/filter.rs
rust/arrow/src/csv/writer.rs
rust/datafusion/src/bin/main.rs
go/arrow/ipc/file_reader.go
js/src/builder/index.ts
... and 280 more

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[wesm]

Rebased. The Status from get_schema wasn't being checked (causing my clang build to fail) so fixed that

[wesm]

+1