Recognize platform.system() as well as sys.platform

Note: if you are reporting a wrong signature of a function or a class in
the standard library, then the typeshed tracker is better suited
for this report: https://github.com/python/typeshed/issues

Please provide more information to help us understand the issue:

• Are you reporting a bug, or opening a feature request? feature request
• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

    if platform.system() != "Windows":  # sys.platform != "win32":
        try:
            # os.confstr("CS_GNU_LIBC_VERSION") returns a string like "glibc 2.17".
            version_string = os.confstr("CS_GNU_LIBC_VERSION")
            assert version_string is not None
            _, version = version_string.split()
        except (AssertionError, OSError, ValueError):
            # os.confstr() or CS_GNU_LIBC_VERSION not available (or a bad value)...
            return None
        return version
    return None

• What is the actual behavior/output? "error: Module has no attribute "confstr""
• What is the behavior/output you expect? No error when run under Windows
• What are the versions of mypy and Python you are using? mypy 0.750, Python 3.7.4

Basically it would be nice to get to use platform.system() instead of having to use sys.platform to make mypy happy with code that is conditional on the platform.

从提供的 Python repro 开始，比较 mypy 在条件代码中如何处理 platform.system() 与 sys.platform。确定特定于平台的分析入口点，并通过检查 os.confstr 在 Windows 分支中未被报告为缺失来验证更改；payload 未命名源文件或测试。