Remove `six` package dependency

[Laerte]

We don't need the bindings since we support python 3.6+.

@jpmckinney also: Can i open another PR to drop support of python 3.6 aswell?

[jpmckinney]

Yes, can do!

[codecov[bot]]

Codecov Report

Merging #458 (9655005) into master (c9cacd3) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #458   +/-   ##
=======================================
  Coverage   81.86%   81.87%           
=======================================
  Files          22       22           
  Lines        1009     1004    -5     
=======================================
- Hits          826      822    -4     
+ Misses        183      182    -1     

Flag	Coverage Δ
unittests	81.87% <100.00%> (+<0.01%)	:arrow_up:

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

Impacted Files	Coverage Δ
setup.py	0.00% <ø> (ø)
scrapyd/environ.py	82.22% <100.00%> (ø)
scrapyd/poller.py	96.55% <100.00%> (-0.12%)	:arrow_down:
scrapyd/utils.py	88.59% <100.00%> (+0.46%)	:arrow_up:
scrapyd/website.py	91.17% <100.00%> (ø)

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