Fix Value and Error inheritance on Python 3.5+

[RazerM]

Fixes #17

[codecov]

Codecov Report

Merging #18 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #18   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           8      8           
  Lines         226    229    +3     
  Branches       12     12           
=====================================
+ Hits          226    229    +3

Impacted Files	Coverage Δ
tests/test_sync.py	100% <100%> (ø)	⬆️
src/outcome/_async.py	100% <100%> (ø)	⬆️

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[njsmith]

Huh, I hadn't seen the hoops you were jumping through to support Python 2 here before. That's clever! Though complex enough that it makes me wonder whether supporting Python 2 is worth it :-).

Anyway, this fix looks good to me!