BF: Fix string literal comparison warning

[jhlegarreta]

Fix string literal comparison warning.

Fixes

/home/vsts/work/1/s/venv/lib/python3.8/site-packages/dipy/reconst/qtdmri.py:157:
SyntaxWarning: "is not" with a literal. Did you mean "!="?

  if laplacian_weighting is not 'GCV':

/home/vsts/work/1/s/venv/lib/python3.8/site-packages/dipy/reconst/qtdmri.py:175:
SyntaxWarning: "is not" with a literal. Did you mean "!="?

  if l1_weighting is not 'CV'

raised for example in:
https://dev.azure.com/dipy/dipy/_build/results?buildId=399&view=logs&j=d9aefae6-c332-51b5-d933-a85009f5a2ac&t=10ba5eab-67e4-5f58-2aec-60b4dbb37e45&l=1926

[codecov]

Codecov Report

Merging #2162 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2162   +/-   ##
=======================================
  Coverage   91.28%   91.28%           
=======================================
  Files         251      251           
  Lines       32413    32421    +8     
  Branches     3419     3420    +1     
=======================================
+ Hits        29587    29595    +8     
  Misses       2085     2085           
  Partials      741      741           

Impacted Files	Coverage Δ
dipy/reconst/tests/test_multi_voxel.py	99.19% <ø> (-0.02%)	⬇️
dipy/reconst/qtdmri.py	93.94% <100.00%> (ø)
dipy/segment/bundles.py	89.09% <100.00%> (ø)
dipy/segment/tests/test_clustering.py	99.38% <100.00%> (ø)
dipy/workflows/stats.py	85.76% <100.00%> (+0.05%)	⬆️
dipy/reconst/tests/test_sfm.py	98.77% <0.00%> (+0.05%)	⬆️
dipy/reconst/sfm.py	92.22% <0.00%> (+0.08%)	⬆️

[skoudoro]

thank you @jhlegarreta