WIP: generalize class for opening image

[matthew-brett]

Allow FileHolder subclasses to change the ImageOpener class.

I wanted to do this to work with the fsspec file objects.

But - I couldn't see immediately what the right way to do the type checking is

• I was considering using OpenDef from openers.py, but that is only defined
  if ty.TYPE_CHECKING is True. I'd love some advice!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.26%. Comparing base (c445edd) to head (251c899).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1314   +/-   ##
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  Coverage   92.26%   92.26%           
=======================================
  Files          99       99           
  Lines       12472    12473    +1     
  Branches     2566     2566           
=======================================
+ Hits        11507    11508    +1     
  Misses        642      642           
  Partials      323      323           

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[effigies]

• I was considering using OpenDef from openers.py, but that is only defined
  if ty.TYPE_CHECKING is True. I'd love some advice!

We could pull that out of the ty.TYPE_CHECKING block. The main point of putting it in there is that it was only used internally and it didn't seem usable at runtime. If you're also only using it as a type annotation, then it will be available to import into other modules like:

if ty.TYPE_CHECKING:
    from nibabel.openers import OpenerDef