Ensure async functionality does not cause python2 syntax errors

[philippjfr]

No description provided.

[codecov-commenter]

Codecov Report

Merging #624 (9f2f037) into master (b78f700) will increase coverage by 1.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #624      +/-   ##
==========================================
+ Coverage   81.79%   82.81%   +1.01%     
==========================================
  Files           4        5       +1     
  Lines        3027     3037      +10     
==========================================
+ Hits         2476     2515      +39     
+ Misses        551      522      -29     

Impacted Files	Coverage Δ
param/_async.py	100.00% <100.00%> (ø)
param/parameterized.py	83.32% <100.00%> (+1.43%)	⬆️
numbergen/__init__.py	80.32% <0.00%> (+0.40%)	⬆️

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[maximlt]

This looks good to me. I'll try to add some tests because I think these code paths aren't tested at all.

[maximlt]

@philippjfr this is ready to me, the tests I've added are really simple and are just meant to get 100% coverage of the _async module.