explicit .use_asyncio() in right spot

[meejah]

No description provided.

[codecov-io]

Current coverage is 61.24%

Merging #560 into master will increase coverage by +0.03% as of 2f72d71

@@            master    #560   diff @@
======================================
  Files           58      58       
  Stmts         9195    9201     +6
  Branches      1481    1482     +1
  Methods          0       0       
======================================
+ Hit           5629    5635     +6
  Partial        496     496       
  Missed        3070    3070       

Review entire Coverage Diff as of 2f72d71

Powered by Codecov. Updated on successful CI builds.

[meejah]

The "base-class" unit-tests needed to select a txaio subsystem (but, this was a bit order-dependent on which tests ran first). Works locally and on travis. I also re-ran all the examples in python3.