Explicit character encoding for all calls to open()

[Veldhoen]

Issue #262 - Character encoding (Windows)

[codecov]

Codecov Report

Merging #263 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #263   +/-   ##
=======================================
  Coverage   99.31%   99.31%           
=======================================
  Files          52       52           
  Lines        2629     2629           
=======================================
  Hits         2611     2611           
  Misses         18       18

Impacted Files	Coverage Δ
annif/project.py	100% <100%> (ø)	⬆️
annif/cli.py	99.43% <100%> (ø)	⬆️
annif/backend/fasttext.py	98.63% <100%> (ø)	⬆️
annif/corpus/subject.py	100% <100%> (ø)	⬆️
annif/corpus/document.py	100% <100%> (ø)	⬆️
annif/corpus/convert.py	100% <100%> (ø)	⬆️
annif/backend/vw_multi.py	100% <100%> (ø)	⬆️

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[osma]

Had to fix the indent to make the PEP8 check pass.
Merged, thanks a lot!