Use zfec wheel package from testpypi

[sajith]

Testing that Tahoe-LAFS can use zfec wheel packages.

[codecov-commenter]

Codecov Report

Merging #804 into master will decrease coverage by 0%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #804    +/-   ##
=======================================
- Coverage      92%     91%    -0%     
=======================================
  Files         155     155            
  Lines       27355   27355            
  Branches     4099    3896   -203     
=======================================
- Hits        25138   25011   -127     
- Misses       1534    1601    +67     
- Partials      683     743    +60     

Impacted Files	Coverage Δ
src/allmydata/util/humanreadable.py	65% <0%> (-16%)	⬇️
src/allmydata/crypto/__init__.py	86% <0%> (-14%)	⬇️
src/allmydata/crypto/error.py	89% <0%> (-11%)	⬇️
src/allmydata/util/_python3.py	89% <0%> (-11%)	⬇️
src/allmydata/storage/common.py	89% <0%> (-11%)	⬇️
src/allmydata/util/assertutil.py	89% <0%> (-11%)	⬇️
src/allmydata/web/common_py3.py	82% <0%> (-9%)	⬇️
src/allmydata/crypto/util.py	92% <0%> (-8%)	⬇️
src/allmydata/util/mathutil.py	92% <0%> (-8%)	⬇️
src/allmydata/util/base32.py	93% <0%> (-7%)	⬇️
... and 41 more

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[sajith]

Closing, because this was meant to be an experimental branch to test if zfec wheel packages can actually work without having to install vcpython27 on Windows. This is superseded by PR #862 which uses zfec from pypi.org.