Fix flake8 'imported but unused' reports
...
$ flake8 | grep F401
./meson.py:17:1: F401 'mesonbuild.mesonlib' imported but unused
./meson.py:18:1: F401 'locale' imported but unused
./run_unittests.py:24:1: F401 'sys' imported but unused
./mesonbuild/minit.py:2:1: F401 'pyclbr.Function' imported but unused
./mesonbuild/minit.py:18:1: F401 'os' imported but unused
./mesonbuild/backend/vs2010backend.py:15:1: F401 'sys' imported but unused
./mesonbuild/backend/xcodebackend.py:19:1: F401 'sys' imported but unused
./mesonbuild/dependencies/ui.py:20:1: F401 'shutil' imported but unused
./mesonbuild/modules/python3.py:15:1: F401 'sys' imported but unused
./mesonbuild/modules/unstable_icestorm.py:15:1: F401 '..compilers' imported but unused
./mesonbuild/modules/unstable_icestorm.py:15:1: F401 '..mlog' imported but unused
./test cases/common/98 gen extra/srcgen3.py:3:1: F401 'os' imported but unused