Suppress ImportError for winreg as the module is only available on some...

Suppress ImportError for winreg as the module is only available on some platforms. Allows unit testing of module on non-Windows platforms.

distutils/_msvccompiler.py

@@ -15,7 +15,9 @@ for older versions in distutils.msvc9compiler and distutils.msvccompiler.
 
 import os
 import subprocess
-import winreg
+import contextlib
+with contextlib.suppress(ImportError):
+    import winreg
 
 from distutils.errors import DistutilsExecError, DistutilsPlatformError, \
                              CompileError, LibError, LinkError