StringIO imports differently in python 3 - use six for compatibility

[chmoder]

cStringIO does not exist in python3, we would like to use this module in python 3. Thanks!

[codecov-io]

Codecov Report

Merging #46 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master      #46   +/-   ##
=======================================
  Coverage   54.08%   54.08%           
=======================================
  Files         125      125           
  Lines       14822    14822           
  Branches     2500     2500           
=======================================
  Hits         8016     8016           
- Misses       6262     6263    +1     
+ Partials      544      543    -1

Flag	Coverage Δ
#py27	52.98% <0%> (ø)	⬆️
#py35	52.33% <0%> (ø)	⬆️
#py36	52.33% <0%> (ø)	⬆️
#py37	52.32% <0%> (ø)	⬆️
#pypy	52.25% <0%> (ø)	⬆️

Impacted Files	Coverage Δ
paste/cascade.py	0% <0%> (ø)	⬆️
paste/progress.py	0% <0%> (ø)	⬆️

---

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 79e5b38...a31377b. Read the comment docs.

[cdent]

thanks, will get a release out shortly