Issue #14574: Ignore socket errors raised when flushing a connection on close.

Issue #14574: Ignore socket errors raised when flushing a connection on close.

This issue is now fixed due to changes in Issue #11939, so I've refactored
the tests to cover the hard link case. There are no code changes here.

By setting the st_dev attribute, we can then remove some Windows-specific
code and move os.path.samefile/sameopenfile/samestat to Lib/genericpath.py
so all platforms share the same implementation.

objects as the base by using PyNumber_AsSsize_t similar to round().

This fixes the following warning when building the docs (probably from
the commit for issue #14901):

"/Doc/faq/windows.rst:138: WARNING: undefined label: setting-envvars (if the
link has no caption the label must precede a section header)"

This adds to the work done for issue #15990 (i.e. f44b8d69e5fc and the
commits referenced there).

This adds to the work done for issue #15990 (i.e. f44b8d69e5fc and the
commits referenced there).

This adds to the work done for issue #15990 (i.e. f44b8d69e5fc and the
commits referenced there).

Consistent with the behavior in Python 2.