Fix loc with MultiIndex

[devin-petersohn]

What do these changes do?

Handles correctly the case where the index is a MultiIndex

Related issue number

Resolves #504

• passes flake8 modin
• passes black --check modin
• tests added and passing

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Codecov Report

Merging #508 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #508      +/-   ##
==========================================
+ Coverage   90.12%   90.17%   +0.05%     
==========================================
  Files          35       35              
  Lines        4828     4836       +8     
==========================================
+ Hits         4351     4361      +10     
+ Misses        477      475       -2

Impacted Files	Coverage Δ
modin/pandas/indexing.py	80.76% <100%> (+1.91%)	⬆️
...management/query_compiler/pandas_query_compiler.py	90.51% <100%> (ø)	⬆️

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[williamma12]

Thanks for the quick catch @devin-petersohn! Just some questions, which might be good places to include inline docs for

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[williamma12]

Looks good! Let's wait for the tests to pass before merging