BUG: Can't get period code with frequency alias 'minute' or 'Minute'
- [x] closes #11854 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Also, added `NS` to normal alias not to show warnings (as the same as `MS` and `NS`)

thank you sir!
