Remove Python 2.6 compatibility code from scripts in buildscripts/ directory

The following files have Python 2.6 compatibility code which we can remove once we have Python 2.7 in the toolchain:

• buildscripts/buildlogger.py
• buildscripts/burn_in_tests.py
• buildscripts/clang_format.py
• buildscripts/eslint.py
• buildscripts/hang_analyzer.py

We should consider fixing this on older branches, when reasonable.