Fix NotEqual position issue

[jimmylai]

Summary

PositionProvider provides inconsistent NotEqual (whitespaces are included) compare to Equal (whitespace is not included). Issue was reported in #315.
Not sure why NotEqual was implemented differently.
Use _BaseOneTokenOp to keep the codegen consistent and add test cases.

Test Plan

Add new test cases.

[codecov-commenter]

Codecov Report

Merging #325 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #325   +/-   ##
=======================================
  Coverage   93.98%   93.99%           
=======================================
  Files         225      225           
  Lines       21778    21787    +9     
=======================================
+ Hits        20469    20478    +9     
  Misses       1309     1309           

Impacted Files	Coverage Δ
libcst/_nodes/op.py	95.66% <100.00%> (-0.03%)	⬇️
libcst/metadata/tests/test_position_provider.py	100.00% <100.00%> (ø)

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[zsol]

lgtm