gh-102475: Fix os.path.realpath() for names which look like a drive

[serhiy-storchaka]

The documentation says that in the non-strict mode the unresolved part of the path is appended, but it was joined with join(), which resets the path if the tail looks like a drive:

>>> ntpath.realpath('C:/spam/a:b')
'a:b'

It is now simply appended, using the naive join suggested by @eryksun.

realpath() also used join(cwd, path) for a relative path, which left a path relative to another drive unresolved. It now uses abspath(), so such path is resolved against the root directory of that drive, as GetFullPathNameW() does:

>>> ntpath.realpath('Z:spam')   # Z: does not exist
'Z:spam'                        # now 'Z:\\spam'

• Issue: ntpath.realpath() mishandles filenames that resemble drives #102475