BF: Use equality check instead of identity check

[jhlegarreta]

Use equality check instead of identity check.

Fixes for example

/home/vsts/work/1/s/venv/lib/python3.8/site-packages/dipy/viz/regtools.py:227:
SyntaxWarning: "is" with a literal. Did you mean "=="?
if direct_grid2world is -1:

Reported for example in
https://dev.azure.com/dipy/46625eaf-7255-4a2c-9e92-befcdbdca2a8/_apis/build/builds/310/logs/267

[skoudoro]

LGTM, Thanks @jhlegarreta.

Waiting for the CI's since I have a tiny small doubt for the workflows change.

[codecov]

Codecov Report

Merging #2139 into master will increase coverage by 0.01%.
The diff coverage is 91.95%.

@@            Coverage Diff             @@
##           master    #2139      +/-   ##
==========================================
+ Coverage   91.20%   91.21%   +0.01%     
==========================================
  Files         251      251              
  Lines       32214    32229      +15     
  Branches     3401     3403       +2     
==========================================
+ Hits        29382    29399      +17     
  Misses       2091     2091              
+ Partials      741      739       -2     

Impacted Files	Coverage Δ
dipy/align/imwarp.py	98.39% <ø> (ø)
dipy/align/scalespace.py	92.64% <ø> (ø)
dipy/viz/regtools.py	31.49% <0.00%> (ø)
dipy/io/stateful_tractogram.py	68.85% <50.00%> (ø)
dipy/core/gradients.py	90.47% <88.88%> (+0.04%)	⬆️
dipy/nn/model.py	92.59% <93.75%> (ø)
dipy/nn/tests/test_tf.py	88.46% <93.75%> (ø)
dipy/core/tests/test_gradients.py	98.76% <100.00%> (ø)
dipy/direction/peaks.py	83.41% <100.00%> (ø)
...ection/tests/test_closest_peak_direction_getter.py	100.00% <100.00%> (ø)
... and 16 more

[skoudoro]

Thank you @jhlegarreta