Use an explicit encoding="utf-8" in every call to open

[mwouts]

Closes #770

[codecov]

Codecov Report

Merging #774 (eba0f41) into master (be5f4aa) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #774   +/-   ##
=======================================
  Coverage   99.11%   99.11%           
=======================================
  Files         106      106           
  Lines       10032    10032           
=======================================
  Hits         9943     9943           
  Misses         89       89           

Impacted Files	Coverage Δ
jupytext/contentsmanager.py	97.85% <ø> (ø)
jupytext/cli.py	96.13% <100.00%> (ø)
jupytext/config.py	97.28% <100.00%> (ø)

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