Fix flake8 'imported but unused' reports

$ flake8 | grep F401
./mesonbuild/compilers/d.py:19:1: F401 '.compilers.GCC_CYGWIN' imported but unused
./test cases/unit/35 dist script/replacer.py:3:1: F401 'sys' imported but unused

mesonbuild/compilers/d.py

@@ -18,7 +18,6 @@ from ..mesonlib import EnvironmentException, version_compare, is_windows, is_osx
 
 from .compilers import (
     GCC_STANDARD,
-    GCC_CYGWIN,
     GCC_OSX,
     d_dmd_buildtype_args,
     d_gdc_buildtype_args,

test cases/unit/35 dist script/replacer.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python3
 
-import os, sys
+import os
 import pathlib
 
 source_root = pathlib.Path(os.environ['MESON_DIST_ROOT'])