Update auth_tkt.py for python3 compatibility

[marzetas]

First of all, thanks for taking care of paste, even if it's just in maintenance mode.

I've been trying to use the auth token module on python3, but I've found an issue with some imports from urllib, which have been moved to a different path in python3.

This PR uses a try..except approach to do the import correctly regardless of the python version.

[codecov-io]

Codecov Report

Merging #45 into master will increase coverage by 0.54%.
The diff coverage is 92.3%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
+ Coverage   54.08%   54.63%   +0.54%     
==========================================
  Files         125      126       +1     
  Lines       14822    14858      +36     
  Branches     2500     2505       +5     
==========================================
+ Hits         8016     8117     +101     
+ Misses       6262     6193      -69     
- Partials      544      548       +4

Flag	Coverage Δ
#py27	53.45% <71.15%> (+0.47%)	⬆️
#py35	52.81% <75%> (+0.47%)	⬆️
#py36	52.81% <75%> (+0.47%)	⬆️
#py37	52.8% <75%> (+0.47%)	⬆️
#pypy	52.72% <71.15%> (+0.47%)	⬆️

Impacted Files	Coverage Δ
tests/test_auth/test_auth_tkt.py	100% <100%> (ø)
paste/auth/auth_tkt.py	39.78% <84%> (+39.78%)	⬆️

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[cdent]

Thanks for doing this. I'll try to get it reviewed and merged some time this week.

[marzetas]

I've found some other issues around string encodings in calculate_digest as well. I'll try to get those sorted as well.