import-checker: ensure multiprocessing is treated as from stdlib

On my linux machines multiprocessing appears to defeat the logic in
import-checker to detect stdlib modules. Since we now only use
versions of Python which ship with multiprocessing, let's just
whitelist the module.

contrib/import-checker.py

@@ -164,7 +164,7 b' def list_stdlib_modules():'
     for m in ['msvcrt', '_winreg']:
         yield m
     # These get missed too
-    for m in 'ctypes', 'email':
+    for m in 'ctypes', 'email', 'multiprocessing':
         yield m
     yield 'builtins' # python3 only
     for m in 'fcntl', 'grp', 'pwd', 'termios':  # Unix only