Run tests on Python 3.6

[johanvdw]

Python 3.6 is the version used in upcoming debian and ubuntu releases.

Note I only changed the tests - I have not yet tested whether this works.

[codecov]

Codecov Report

Merging #565 into master will increase coverage by 0.21%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #565      +/-   ##
==========================================
+ Coverage   55.81%   56.03%   +0.21%     
==========================================
  Files          28       28              
  Lines        6355     6882     +527     
  Branches     1349     1566     +217     
==========================================
+ Hits         3547     3856     +309     
- Misses       2423     2619     +196     
- Partials      385      407      +22

Flag	Coverage Δ
#integrationtests	54.43% <ø> (-0.03%)	⬇️
#unittests	8.8% <ø> (+1.22%)	⬆️

Impacted Files	Coverage Δ
pycsw/server.py	64.56% <0%> (+0.2%)	⬆️
pycsw/core/util.py	91.07% <0%> (+0.83%)	⬆️
pycsw/core/metadata.py	26.01% <0%> (+3.7%)	⬆️
pycsw/core/repository.py	68.2% <0%> (+7.37%)	⬆️
pycsw/core/admin.py	36.58% <0%> (+12.1%)	⬆️

---

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 7845e90...036ccc8. Read the comment docs.

[tomkralidis]

Thanks @johanvdw!

[kalxas]

Thanks @johanvdw