Apply pyupgrade suggestions

[DimitriPapadopoulos]

• In Python 3, string literals are Unicode by default.
• Use yield from instead of a loop with yield.
• Delete empty 1st line.
• In subprocess.check_output arguments., replace universal_newlines with text.

I tried to replace stdout=subprocess.PIPE/stderr=subprocess.PIPE with capture_output=True, but it doesn't work. Yet capture_output is documented to be available in Python >= 3.7:
https://docs.python.org/3/library/subprocess.html#subprocess.run

[arokem]

LGTM!

[codecov]

Codecov Report

Merging #2891 (dca4498) into master (a46138c) will decrease coverage by 0.02%.
The diff coverage is 66.66%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2891      +/-   ##
==========================================
- Coverage   81.41%   81.40%   -0.02%     
==========================================
  Files         145      145              
  Lines       20147    20146       -1     
  Branches     3217     3216       -1     
==========================================
- Hits        16402    16399       -3     
- Misses       2929     2931       +2     
  Partials      816      816              

Files Changed	Coverage Δ
dipy/align/__init__.py	100.00% <ø> (ø)
dipy/core/ndindex.py	83.33% <0.00%> (+6.41%)	⬆️
dipy/core/sphere.py	94.04% <ø> (ø)
dipy/denoise/gibbs.py	97.39% <ø> (ø)
dipy/denoise/nlmeans.py	100.00% <ø> (ø)
dipy/denoise/noise_estimate.py	78.88% <ø> (ø)
dipy/denoise/non_local_means.py	100.00% <ø> (ø)
dipy/direction/__init__.py	100.00% <ø> (ø)
dipy/io/bvectxt.py	37.50% <ø> (ø)
dipy/io/image.py	76.31% <ø> (ø)
... and 19 more

... and 1 file with indirect coverage changes

[skoudoro]

Thanks @DimitriPapadopoulos,

I am going ahead and merge this PR