remove use of deprecated ioutil package

[sebastien-rosset]

Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features) see docs/README.md

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

This is removing use of ioutil package which is deprecated.

What is the current behavior? (You can also link to an open issue here)

No change.

What is the new behavior (if this is a feature change)?

No change.

Does this PR introduce a breaking change? (What changes might users need to make due to this PR?)

No

Other information: