[appveyor] Fix coverage by installing sdist version

[pquentin]

This is closer to what we do in Travis. However, async_generator.__file__ was not pointing to the correct path (shown by pip show -f async_generator), so I decided to build it manually. This is probably quite brittle, but at least it currently works.

Fixes #6.

[codecov]

Codecov Report

Merging #7 into master will decrease coverage by 8.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master       #7      +/-   ##
==========================================
- Coverage     100%   91.91%   -8.09%     
==========================================
  Files           5        5              
  Lines         581      581              
  Branches       48       48              
==========================================
- Hits          581      534      -47     
- Misses          0       47      +47

Impacted Files	Coverage Δ
async_generator/__init__.py	0% <0%> (-100%)	⬇️
async_generator/util.py	42.85% <0%> (-57.15%)	⬇️
async_generator/impl.py	74.17% <0%> (-25.83%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e303e07...c7909c9. Read the comment docs.

[pquentin]

I'm not sure why the coverage is lower. And it looks like there's work left for the codecov integration in Appveyor.

[pquentin]

Closing as I'm not sure this improves anything. If someone else comes along maybe they'll be able to continue that work.