Fix AuthBasicAuthenticator base64 decoding to work on py3

[codecov-io]

Codecov Report

Merging #61 into master will decrease coverage by 0.00%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
- Coverage   54.78%   54.78%   -0.01%     
==========================================
  Files         126      126              
  Lines       14901    14903       +2     
  Branches     2510     2510              
==========================================
  Hits         8164     8164              
- Misses       6178     6180       +2     
  Partials      559      559              

Flag	Coverage Δ
#py27	53.62% <0.00%> (-0.01%)	:arrow_down:
#py35	52.98% <0.00%> (-0.01%)	:arrow_down:
#py36	52.98% <0.00%> (-0.01%)	:arrow_down:
#py37	52.98% <0.00%> (-0.01%)	:arrow_down:
#pypy	52.88% <0.00%> (-0.01%)	:arrow_down:

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Impacted Files	Coverage Δ
paste/auth/basic.py	0.00% <0.00%> (ø)

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[cdent]

thanks!