Enable Python version auto-detection for Black formatter

Author: ivankravets

Makefile

@@ -7,8 +7,8 @@ isort:
 isort ./tests
 
 format:
-black --target-version py27 ./platformio
-black --target-version py27 ./tests
+black ./platformio
+black ./tests
 
 test:
 py.test --verbose --capture=no --exitfirst -n 6 --dist=loadscope tests --ignore tests/test_examples.py