appveyor.yml: try different artifact syntax

[warner]

With luck this will upload each binary .whl file as a separate "artifact"
file, rather than merging them all together into a single dist.zip bundle.

[codecov-io]

Codecov Report

Merging #466 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #466      +/-   ##
==========================================
- Coverage   93.02%   93.01%   -0.01%     
==========================================
  Files         232      232              
  Lines       58668    58668              
  Branches     7649     7649              
==========================================
- Hits        54573    54571       -2     
- Misses       3126     3127       +1     
- Partials      969      970       +1

Impacted Files	Coverage Δ
src/allmydata/mutable/servermap.py	93.38% <0%> (-0.49%)	⬇️
src/allmydata/test/test_system.py	94.89% <0%> (-0.07%)	⬇️
src/allmydata/test/test_introducer.py	98.53% <0%> (+0.29%)	⬆️

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