Fix DeprecationWarning: invalid escape sequence in versioneer.py
Hello,

This is a little patch to fix a `DeprecationWarning: invalid escape sequence` in `versioneer.py`.

- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
