[MRG] Drop __future__ imports

[standage]

Now that we've dropped Python 2 support, __future__ imports are unnecessary. Aside from the Makefile, all changes to the code in this PR were made by grep -v and shell for loops.

NB: Splitting this out from #1774 to simplify code review.

• Is it mergeable?
• make test Did it pass the tests?
• make clean diff-cover If it introduces new functionality in
  scripts/ is it tested?
• make format diff_pylint_report cppcheck doc pydocstyle Is it well
  formatted?
• For substantial changes or changes to the command-line interface, is it
  documented in CHANGELOG.md? See keepachangelog
  for more details.

[standage]

I'll handle merging this back into #1774, including conflicts if there are any.

[standage]

Ready for review @ctb @luizirber @betatim!

[codecov-io]

Codecov Report

Merging #1786 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #1786      +/-   ##
=========================================
- Coverage    0.06%   0.05%   -0.02%     
=========================================
  Files          78      78              
  Lines        9792    9757      -35     
  Branches     2457    2457              
=========================================
- Hits            6       5       -1     
+ Misses       9786    9752      -34

Impacted Files	Coverage Δ
khmer/kfile.py	0% <ø> (ø)	⬆️
scripts/sample-reads-randomly.py	0% <ø> (ø)	⬆️
scripts/partition-graph.py	0% <ø> (ø)	⬆️
scripts/merge-partitions.py	0% <ø> (ø)	⬆️
scripts/filter-stoptags.py	0% <ø> (ø)	⬆️
khmer/khmer_logger.py	0% <ø> (ø)	⬆️
scripts/find-knots.py	0% <ø> (ø)	⬆️
khmer/trimming.py	0% <ø> (ø)	⬆️
scripts/normalize-by-median.py	0% <ø> (ø)	⬆️
scripts/unique-kmers.py	0% <ø> (ø)	⬆️
... and 24 more

---

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[camillescott]

Looks fine to me!

[betatim]

Before I hit merge, do you want to merge this into master or #1774? The PR says master but your comment makes me wonder.

[standage]

Definitely into master. It's a subset of changes from #1774, which I'm trying to break into smaller more manageable chunks. I'll resolve any conflicts on that branch that arise from merging this PR.