Add a constraint on lxml version to avoid an error when using Python 3.4

[Gallaecio]

Based on scrapy/scrapy#3918

[codecov]

Codecov Report

Merging #145 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #145   +/-   ##
=======================================
  Coverage   99.64%   99.64%           
=======================================
  Files           5        5           
  Lines         279      279           
  Branches       51       51           
=======================================
  Hits          278      278           
  Misses          1        1

---

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[codecov]

Codecov Report

Merging #145 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #145   +/-   ##
=======================================
  Coverage   99.64%   99.64%           
=======================================
  Files           5        5           
  Lines         279      279           
  Branches       51       51           
=======================================
  Hits          278      278           
  Misses          1        1

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fafba41...ec8fc19. Read the comment docs.

[kmike]

👍