BUG: Add uint64 support to IntervalTree
- [X] xref https://github.com/pandas-dev/pandas/issues/20636#issuecomment-380276144
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

Note that this doesn't fully close the linked issue; this just resolves the `uint64` case, which was relatively straightforward.  The datetimelike case still needs to be resolved, but seems distinct enough for a different PR.