Fix delim_whitespace behavior in read_table, read_csv
- [x] closes #36583
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @OlehKSS! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-10-07 21:31:37 UTC cc @gfyoung if you can have a look. The changes look pretty reasonable, @jreback's comment notwithstanding.

Test failures look unrelated AFAICT. I updated the pull request:

- added a note to whatsnew
- moved duplicated validation to a separate function @jreback @gfyoung @phofl 
I implemented the requested changes in the latest commits. Could you take one more look? lgtm, ping on green.

cc @gfyoung if any comments. The changes look good, but not sure why CI isn't all green... One CI test is failing because it exceeds a timeout of 60min (Windows_py38_np18) while running tests. It seems like there are no tests that actually crash, but one test results in [Windows Access Violation](https://dev.azure.com/pandas-dev/e01c9671-576d-40f3-a1a1-7bef78604087/_apis/build/builds/44454/logs/120) in one of the threads, which seems raised by a dependency thanks @OlehKSS very nice.