Fix nightly yapf warnings + minor refactoring

[sdimitro]

Today's nightly failed because the new version of yapf changed its formatting for a specific form.
The nightly run: https://github.com/delphix/sdb/runs/613792165?check_suite_focus=true

This commit applies the new yapf style.

Some minor refactoring are added as a side-change in this commit:
[1] 1 unused variable (err in per_cpu.py) is removed
[2] 2 unused imports are removed (List in zfs/internal/__init__.py and find_packages in setup.py
[3] 3 range() refactorings (2 range(0, X) to range(X) and one range() -> enumerate())

[codecov-io]

Codecov Report

Merging #213 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
- Coverage   86.76%   86.75%   -0.01%     
==========================================
  Files          59       59              
  Lines        2402     2401       -1     
==========================================
- Hits         2084     2083       -1     
  Misses        318      318              

Impacted Files	Coverage Δ
sdb/commands/linux/slabs.py	95.12% <ø> (ø)
sdb/commands/spl/spl_kmem_caches.py	88.75% <ø> (ø)
sdb/commands/zfs/internal/__init__.py	96.87% <ø> (-0.10%)	⬇️
sdb/pipeline.py	85.36% <ø> (ø)
sdb/commands/internal/table.py	93.33% <100.00%> (ø)
sdb/commands/linux/per_cpu.py	100.00% <100.00%> (ø)
sdb/commands/zfs/metaslab.py	95.50% <100.00%> (ø)
sdb/commands/zfs/vdev.py	89.28% <100.00%> (ø)

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