devnull open mode fix + minor optimizations

[v1k1nghawk]

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Description

1. it's enough to open os.devnull using 'w' mode instead of 'rw';
2. logging optimized based on lazy string interpolation (https://docs.python.org/2/howto/logging.html#optimization);
3. stderr_data variable removed where not used.

💔Thank you!

[CLAassistant]

All committers have signed the CLA.

[xdanx]

Approved, thanks for the contribution!