chore: fix log.py from failing lint-3.6 check

[Neenu1995]

lint-3.6 check was failing in master branch of synthtool with the following error.

synthtool/log.py:21: error: Cannot assign to a type
synthtool/log.py:21: error: Incompatible types in assignment (expression has type "None", variable has type "Type[ColoredFormatter]")
synthtool/log.py:71: error: Incompatible types in assignment (expression has type "Formatter", variable has type "ColoredFormatter")
Found 3 errors in 1 file (checked 47 source files)
nox > Command mypy synthtool autosynth failed with exit code 1
nox > Session lint-3.6 failed.