py-intervaltree: updated to 3.1.0

Version 3.1.0
- Dropped support for Python 3.4, added Python 3.8
- Add `__slots__` optimization in Node class, should give performance improvement
- Fixed:
  - Restore universal wheels
  - Bytes/str type incompatibility in setup.py
  - New version of distutils rejects version suffixes of `.postNN`, use `aNN` instead

(adam)