Make it clear how to write platform-dependent stubs #2275

Background

• These are some platform-dependent stubs
  

  typeshed/stdlib/2/os/__init__.pyi

  Lines 38 to 52 in fa74160

  	O_DSYNC: int # Unix only
  	O_RSYNC: int # Unix only
  	O_SYNC: int # Unix only
  	O_NDELAY: int # Unix only
  	O_NONBLOCK: int # Unix only
  	O_NOCTTY: int # Unix only
  	O_SHLOCK: int # Unix only
  	O_EXLOCK: int # Unix only
  	O_BINARY: int # Windows only
  	O_NOINHERIT: int # Windows only
  	O_SHORT_LIVED: int # Windows only
  	O_TEMPORARY: int # Windows only
  	O_RANDOM: int # Windows only
  	O_SEQUENTIAL: int # Windows only
  	O_TEXT: int # Windows only

• Two different styles are used to write these stubs, and this difference is confusing for contributors
  • Using comments like # Windows only or # Unix only
  • Using sys.platform like

    typeshed/stdlib/2and3/time.pyi

    Line 82 in fa74160

    if sys.platform != 'win32':

Proposal

• Decide a guideline on writing platform-dependent stubs
• Summarize it into the conventions section in CONTRIBUTING.md
• Update the existing stubs with the consistent style

Notes

• Based on this review Add nis stub #2213 (comment) by @gvanrossum, use of sys.platform seems to be recommended now?
• I'm interested in updating existing stubs after a guideline is decided