Port test_introducer.py to Python 3

[itamarst]

Fixes https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3514

[exarkun]

Thanks. Looks pretty good. Some comments inline. Here's N more:

• Do we have a best practice for strings that represent paths? The first two tests in
  test_introducer make two different choices - native string literal and unicode string literal.
• in test_introducer there is a TooNewServer that still uses native strings for its VERSION. It's used by a test that asserts some error has happened. I guess that test is trying to assert that the right error happened but since the real IntroducerService has had its VERSION made into byte strings instead of native strings I wonder if that test is really working right.

[itamarst]

I could not reproduce the Python 3 failure locally, but tried to fix it based on logs. Merged forward, hopefully correctly.

[exarkun]

Thanks. Looks good to me. Just one minor comment inline. Please address and then merge.

[codecov]

Codecov Report

Merging #905 (5872220) into master (fba386c) will increase coverage by 0%.
The diff coverage is 95%.

@@          Coverage Diff           @@
##           master    #905   +/-   ##
======================================
  Coverage      92%     92%           
======================================
  Files         157     157           
  Lines       27419   27436   +17     
  Branches     4087    4093    +6     
======================================
+ Hits        25188   25216   +28     
+ Misses       1569    1560    -9     
+ Partials      662     660    -2     

Impacted Files	Coverage Δ
src/allmydata/util/_python3.py	100% <ø> (ø)
src/allmydata/introducer/common.py	98% <86%> (-2%)	⬇️
src/allmydata/introducer/client.py	91% <94%> (+<1%)	⬆️
src/allmydata/client.py	94% <100%> (+<1%)	⬆️
src/allmydata/introducer/server.py	96% <100%> (+<1%)	⬆️
src/allmydata/mutable/publish.py	97% <100%> (+1%)	⬆️
src/allmydata/web/introweb.py	99% <100%> (ø)
src/allmydata/web/status.py	95% <0%> (+<1%)	⬆️
src/allmydata/util/rrefutil.py	92% <0%> (+12%)	⬆️

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