BUG: rolling not accepting Timedelta-like window args (#15440)
 - [x] closes #15440
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

Allow `rolling()` to accept `pd.Timedelta` or `datetime.timedelta` as a `window` argument.
lgtm. minor change. ping on green. Removed `pd.Timedelta` from `isinstance` check.

Looks like appveyor failed early:

```
Build started
git clone -q https://github.com/pandas-dev/pandas.git C:\projects\pandas-465
fatal: early EOF
fatal: The remote end hung up unexpectedly
fatal: index-pack failed
error: RPC failed; curl 18 transfer closed with outstanding read data remaining
Command exited with code 128
``` yeah its fine. ping on green from travis. Ping travis green @mroeschke Thanks! @mroeschke Thanks for taking a crack at this!