Update test_requirements.txt to resolve pytest-xdist conflict and deprecated pandas .ix

[nicain]

Internal build failing because:

• of a version conflict with pytest and pytest-xdist. This PR will remain open until the internal builds are running again, so that the 0.16.2 whl can be published to pypi
• Running pandas 0.24 has error behavior because of .ix notation

[nicain]

Branch builds passing:

• http://bamboo.corp.alleninstitute.org/browse/IFR-APG5-3
• http://bamboo.corp.alleninstitute.org/browse/IFR-AAG7-2

[codecov-io]

Codecov Report

Merging #591 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #591   +/-   ##
=======================================
  Coverage   44.94%   44.94%           
=======================================
  Files          92       92           
  Lines       11880    11880           
=======================================
  Hits         5340     5340           
  Misses       6540     6540

Impacted Files	Coverage Δ
allensdk/brain_observatory/locally_sparse_noise.py	31.43% <0%> (ø)	⬆️

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[dyf]

maybe double check the session analysis regression test to be safe.

[nicain]

maybe double check the session analysis regression test to be safe.

These tests pass, but the nightly has been failing for some time and I would like to get those passing again before this merge, to see if this causes any regressions

[nicain]

Nighty builds are passing again(!) on commit 2a8d9ef:

http://bamboo.corp.alleninstitute.org/browse/IFR-ANG2-1
http://bamboo.corp.alleninstitute.org/browse/IFR-ANPG2-1