create blank config.py using cross-platform technique

create blank config.py using cross-platform technique

os.mknod() fails on OSX with:

Traceback (most recent call last):

  File "/Users/travis/build/fdroidtravis/fdroidserver/tests/../fdroid", line 154, in <module>
    main()
  File "/Users/travis/build/fdroidtravis/fdroidserver/tests/../fdroid", line 130, in main
    mod.main()
  File "/Users/travis/build/fdroidtravis/fdroidserver/fdroidserver/update.py", line 1768, in main
    common.write_to_config(config, 'repo_keyalias', config['repo_keyalias'])
  File "/Users/travis/build/fdroidtravis/fdroidserver/fdroidserver/common.py", line 2328, in write_to_config
    os.mknod(cfg)
PermissionError: [Errno 1] Operation not permitted