all: remove use of deprecated io/ioutil package

[sbinet]

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[codecov]

Codecov Report

Merging #173 (c398aac) into master (d6a3924) will not change coverage.
The diff coverage is 100.00%.

@@ Coverage Diff @@## master #173 +/- ##
=======================================
Coverage 72.90% 72.90% =======================================
Files 71 71 Lines 11683 11683 =======================================
Hits 8517 8517 Misses 2595 2595 Partials 571 571 

Impacted Files	Coverage Δ
modules/runtime.go	54.88% <100.00%> (ø)
py/file.go	58.77% <100.00%> (ø)
pytest/pytest.go	49.09% <100.00%> (ø)

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