Drop support for legacy Python 2.7

[hugovk]

Fixes #234.

And upgrade Python syntax using https://github.com/asottile/pyupgrade.

[codecov]

Codecov Report

Merging #235 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #235      +/-   ##
==========================================
+ Coverage   97.63%   97.71%   +0.08%     
==========================================
  Files          11       10       -1     
  Lines        1564     1532      -32     
==========================================
- Hits         1527     1497      -30     
+ Misses         37       35       -2

Impacted Files	Coverage Δ
sparse/sparse_array.py	97.22% <100%> (-0.34%)	⬇️
sparse/slicing.py	99.2% <100%> (-0.02%)	⬇️
sparse/coo/umath.py	97.13% <100%> (ø)	⬆️
sparse/coo/core.py	96.7% <100%> (-0.04%)	⬇️
sparse/dok.py	95.53% <100%> (-0.04%)	⬇️
sparse/coo/indexing.py	99.18% <100%> (ø)	⬆️
sparse/utils.py	100% <100%> (ø)	⬆️
sparse/coo/common.py	98.1% <100%> (-0.03%)	⬇️

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[hameerabbasi]

This pull request fixes 4 alerts when merging 6969cd5 into 4b262fb - view on LGTM.com

fixed alerts:

• 4 for Unused import

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Comment posted by LGTM.com

[hameerabbasi]

This looks great, thanks! Let me know when you’re done and I’ll merge.

[hameerabbasi]

Could you also add testing for 3.7?

[hameerabbasi]

I wasn't aware that you needed this, thanks!

[hugovk]

Yes, see travis-ci/travis-ci#9069 / travis-ci/travis-ci#9815 for details (sudo is no longer needed).

[hameerabbasi]

I don't mean to ask for too much, but it seems like there are %-strings in the code for a while now. Is there a way to automatically replace those? (According to the pyupgrade docs, --py3-plus should have done it).

[hugovk]

Maybe pyupgrade only replaces % formatters on explicit strings.

[hameerabbasi]

Understood, I'll merge once the last check passes.

[hameerabbasi]

This pull request fixes 4 alerts when merging ce5b2c6 into 4b262fb - view on LGTM.com

fixed alerts:

• 4 for Unused import

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Comment posted by LGTM.com

[hameerabbasi]

Merging, thanks a lot, @hugovk!