CLN: cleanup up platform / python version checks. fix GB10151
Notes:
- I did not use `is_platform_xxx` on `pandas/util/clipboard.py` because this module is a fork. So I guess better not deviate from the original, right?
- I added the helper `_skip_if_windows` function in `pandas/util/testing.py`, I added an underscore before `skip` to maintain consistency with other `skip` functions (although none of them should have this underscore...)

> I think we could use an update on the wiki on the same 

I didnt find any references on the wiki or docs.
