Shorten selector representations using an ellipsis

[Gallaecio]

Fork of #95 based on the feedback there by @kmike that never got an answer.

Fixes #91.

My Unicode heart asked me for using a single ellipsis character (…) in Python 3, but I guess having a consistent behavior across Python versions is preferred 🙂

I used a regular test function instead of doctests to test a wide array of scenarios without making the documentation too noisy.

I feel like the implementation is overkill. I blame the tests.

[codecov]

Codecov Report

Merging #141 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #141      +/-   ##
==========================================
+ Coverage   99.63%   99.64%   +0.01%     
==========================================
  Files           5        5              
  Lines         271      279       +8     
  Branches       48       51       +3     
==========================================
+ Hits          270      278       +8     
  Misses          1        1

Impacted Files	Coverage Δ
parsel/utils.py	100% <ø> (ø)	⬆️
parsel/selector.py	100% <100%> (ø)	⬆️

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[kmike]

Looks good, thanks @Gallaecio for finishing it, @harshasrinivas for the initial implementation and @eliasdorneles for reviews!

This PR has a risk of breaking some of the Scrapy tests, we'll need to take a look at it, probably when doing the Scrapy release.

[Gallaecio]

I’ve tried running tox -e py36 on scrapy after adding git+https://github.com/scrapy/parsel.git to its deps in tox.ini and tests passed, so we may get lucky 🙂