Thanks Erik for the details. On Mac with master also it works like Cygwin.

./python.exe
Python 3.8.0a0 (heads/master:6f85b826b5, Oct  4 2018, 22:44:36)
[Clang 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> f = open('/dev/null', 'wb')
>>> f.seekable()
True
>>> f.write(b'abcdefgh')
8
>>> f.tell()
8
>>> f.seek(8)
8
>>> f.tell()
8