[TST]: Wrong Corr with Timedelta index
- [x] xref #31286 (closes the issue, if there is no way around the numerical issues)
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`


When using var instead of std, we can solve the numerical issues for the cases where var is still within floating point precision while std would not. ok i'll buy this, why don't you add a bug fix note (in combination with the warning should be sufficient) Thanks, should we close this issue then? Looks like this is still worthwhile. Can you merge in master and fix the merge conflicts? cc @mroeschke Conflicts resolved Sorry could you merge master one more time just to be sure. The travis failure doesn't look related to this PR cc @mroeschke 

Merged master, checks failure unlrelated. Opened #37404 to clean it up cc @mroeschke Travis successful. Merged master to show ci/checks is green now. Looks like there's one more merge conflict Done thanks @phofl 