Bug fix release for the 2.4.x series.

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

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

2.4.1 (Monday 27 May 2019)

Contributions from Egor Pafilov, Jath Palasubramaniam, Richard Nemec, and
Dave Allured.

Enhancements

• Enable mmap, keep_file_open options when loading any
  DataobjImage (pr/759) (CM, reviewed by PM)

Bug fixes

• Ensure loaded GIFTI files expose writable data arrays (pr/750) (CM,
  reviewed by PM)
• Safer warning registry manipulation when checking for overflows (pr/753)
  (CM, reviewed by MB)
• Correctly write .annot files with duplicate lables (pr/763) (Richard Nemec
  with CM)

Maintenance

• Fix typo in coordinate systems doc (pr/751) (Egor Panfilov, reviewed by
  CM)
• Replace invalid MINC1 test file with fixed file (pr/754) (Dave Allured
  with CM)
• Update Sphinx config to support recent Sphinx/numpydoc (pr/749) (CM,
  reviewed by PM)
• Pacify FutureWarning and DeprecationWarning from h5py, numpy
  (pr/760) (CM)
• Accommodate Python 3.8 deprecation of collections.MutableMapping
  (pr/762) (Jath Palasubramaniam, reviewed by CM)

API changes and deprecations

• Deprecate keep_file_open == 'auto' (pr/761) (CM, reviewed by PM)