Build: Python 3 bytes to str fix

[boberfly]

Build: Python 3 has a 'bytes' type so it needs to decode to a utf-8 string for a returning std out subprocess.

Generally describe what this PR will do, and why it is needed.

• Fixes Python 3-based SCons builds. Perhaps the check isn't necessary and a decode can be applied for Python 2?

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Cortex project's prevailing coding style and conventions.
• If my code made breaking changes, I applied the pr-majorVersion label to this PR.