Fix flake8 whitespace reports

$ flake8 | grep -E '(E128|E203|E221|E226|E303|W291|W293)'
./mesonbuild/backend/backends.py:32:1: E303 too many blank lines (3)
./mesonbuild/modules/i18n.py:90:56: E128 continuation line under-indented for visual indent

mesonbuild/backend/backends.py

@@ -28,7 +28,6 @@ from collections import OrderedDict
 import shlex
 
 
-
 class CleanTrees:
     '''
     Directories outputted by custom targets that have to be manually cleaned