Use universal_newlines in subprocess

[nettle]

Describe the task

To make code simpler and more readable we can use universal_newlines=True in subprocess instead of .decode("utf-8")

Scope

1. Add universal_newlines=True in subprocess call, e.g. 'subprocess.Popen', subprocess.run
2. Remove output decoding .decode("utf-8")
3. Make sure nothing is broken

Note that we can potentially use text=True which is self-explainable alias,
but universal_newlines is supported from version 3.3 while text is added in 3.7

Definition of Done

• output decoding .decode("utf-8") is removed
• output is text
• no functionality change (all tests pass)