Fixes pyre type-check errors.

[lisroach]

Summary

New type errors introduced with the new LibCST version. run_command now takes a list, and cst.BaseString shows up in a new place. #195

Test Plan

pyre --preserve-path check

[codecov-commenter]

Codecov Report

Merging #196 (b227638) into master (59c7a3d) will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master     #196   +/-   ##
=======================================
  Coverage   86.05%   86.05%           
=======================================
  Files          91       91           
  Lines        3722     3722           
=======================================
  Hits         3203     3203           
  Misses        519      519           

Impacted Files	Coverage Δ
fixit/common/generate_pyre_fixtures.py	33.33% <40.00%> (ø)
fixit/rules/cls_in_classmethod.py	93.75% <100.00%> (ø)

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