[MRG] add proper newline output for csv module

[ctb]

Opens CSV files with newline='' as specified in Python docs (link).

Fixes #1318.

Unfortunately there seems to be no reliable way to test this PR on non-Windows machines because the output is identical with or without newline=''!

Checklist

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Merging #1319 (19f2671) into latest (77467de) will increase coverage by 5.25%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1319      +/-   ##
==========================================
+ Coverage   89.15%   94.41%   +5.25%     
==========================================
  Files         123       96      -27     
  Lines       18593    14982    -3611     
  Branches     1432     1433       +1     
==========================================
- Hits        16577    14145    -2432     
+ Misses       1780      601    -1179     
  Partials      236      236              

Flag	Coverage Δ
python	94.41% <100.00%> (+<0.01%)	⬆️
rust	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/sourmash/cli/sig/cat.py	100.00% <ø> (ø)
src/sourmash/cli/sig/describe.py	100.00% <ø> (ø)
src/sourmash/cli/sig/split.py	100.00% <ø> (ø)
src/sourmash/cli/sketch/dna.py	100.00% <ø> (ø)
src/sourmash/cli/sketch/protein.py	100.00% <ø> (ø)
src/sourmash/cli/sketch/translate.py	100.00% <ø> (ø)
src/sourmash/commands.py	82.63% <100.00%> (ø)
src/sourmash/lca/command_classify.py	78.75% <100.00%> (ø)
src/sourmash/lca/command_index.py	91.48% <100.00%> (-0.05%)	⬇️
src/sourmash/lca/command_summarize.py	80.80% <100.00%> (ø)
... and 29 more

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[ctb]

Ready for review @bluegenes @luizirber!

[bluegenes]

lgtm, but i don't have a windows machine to test on :)

[ctb]

lol, indeed! barring input from @luizirber I think we'll just have to merge & release it and tag in the creator of the original issue.