exclude broken pytest 3.3.0 release

[ThomasWaldmann]

pytest-dev/pytest#2957

[codecov-io]

Codecov Report

Merging #3407 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3407      +/-   ##
==========================================
- Coverage   86.02%   86.01%   -0.02%     
==========================================
  Files          36       36              
  Lines        9064     9064              
  Branches     1501     1501              
==========================================
- Hits         7797     7796       -1     
- Misses        857      858       +1     
  Partials      410      410

Impacted Files	Coverage Δ
src/borg/archive.py	83.21% <0%> (-0.59%)	⬇️
src/borg/archiver.py	87.96% <0%> (+0.41%)	⬆️

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