Define the unittests using pytest

[kszucs]

Rationale for this change

pytest is more convenient for writing python tests and we should be able to reuse the testing suite define in pyarrow.

What changes are included in this PR?

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

Merging #493 (00ce946) into master (b84789a) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@ Coverage Diff @@## master #493 +/- ##
==========================================
- Coverage 76.09% 76.07% -0.02% 
==========================================
Files 155 156 +1 Lines 26575 26750 +175 ==========================================
+ Hits 20221 20351 +130 - Misses 6354 6399 +45 

Impacted Files	Coverage Δ
datafusion-cli/src/lib.rs	0.00% <0.00%> (ø)
datafusion-cli/src/main.rs	0.00% <0.00%> (ø)
...lista/rust/core/src/serde/logical_plan/to_proto.rs	61.64% <0.00%> (-0.77%)	⬇️
...sta/rust/core/src/serde/logical_plan/from_proto.rs	35.41% <0.00%> (-0.51%)	⬇️
datafusion/src/sql/utils.rs	66.54% <0.00%> (-0.25%)	⬇️
datafusion/src/optimizer/utils.rs	48.05% <0.00%> (-0.18%)	⬇️
...ta/rust/core/src/serde/physical_plan/from_proto.rs	38.51% <0.00%> (-0.14%)	⬇️
datafusion/src/logical_plan/expr.rs	84.56% <0.00%> (-0.05%)	⬇️
datafusion/src/physical_plan/planner.rs	80.19% <0.00%> (ø)
datafusion/src/physical_plan/window_frames.rs	86.60% <0.00%> (ø)
... and 1 more

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[jorgecarleitao]

Looks great!

[jimexist]

see related #484

[alamb]

@kszucs FYI PR now needs to be rebased. I agree btw that pytest is really nice (and much nicer than unittest) 👍

[jorgecarleitao]

Do we need two requirements files? I think we only need one, for the tests?

[kszucs]

We used multiple requirements file in pyarrow for easier dependency composition. Though there are nicer ways to define multiple kinds of dependency. I don't have a strong preference, so I'm going to add pytest to the requiremets.txt file.