Bug fix release for the 2.3 series.

Most work on NiBabel so far has been by Matthew Brett (MB), Michael Hanke (MH)
Ben Cipollini (BC), Marc-Alexandre Côté (MC), Chris Markiewicz (CM), Stephan
Gerhard (SG), Eric Larson (EL), Yaroslav Halchenko (YOH) and Chris Cheng (CC).

References like "pr/298" refer to github pull request numbers.

2.3.3 (Wednesday 16 January 2019)

Maintenance

• Restore six dependency (pr/714) (CM, reviewed by Gael Varoquaux, MB)