[MRG] 100% test coverage and bump to 3.1.0

This brings the test coverage to 100%:

```
make test-coverage
...
Name                     Stmts   Miss Branch BrMiss  Cover   Missing
--------------------------------------------------------------------
root_numpy                   5      0      0      0   100%   
root_numpy._root_numpy     132      0     78      0   100%   
root_numpy._warnings         8      0      0      0   100%   
root_numpy.info              2      0      0      0   100%   
root_numpy.utils            34      0     14      0   100%   
--------------------------------------------------------------------
TOTAL                      181      0     92      0   100%   
----------------------------------------------------------------------
```