BUG: Use manager to set title

[larsoner]

Deal with mpl deprecation:

  File "/home/larsoner/python/nibabel/nibabel/viewers.py", line 122, in __init__
    fig.canvas.set_window_title(str(title))
  File "/home/larsoner/python/matplotlib/lib/matplotlib/cbook/deprecation.py", line 233, in wrapper
    _warn_external(warning)
  File "/home/larsoner/python/matplotlib/lib/matplotlib/cbook/__init__.py", line 2120, in _warn_external
    warnings.warn(message, category, stacklevel)
matplotlib.cbook.deprecation.MatplotlibDeprecationWarning: 
The set_window_title function was deprecated in Matplotlib 3.4 and will be removed two minor releases later. Use manager.set_window_title or GUI-specific methods instead.

[codecov]

Codecov Report

Merging #958 into master will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #958   +/-   ##
=======================================
  Coverage   91.87%   91.87%           
=======================================
  Files          98       98           
  Lines       12451    12451           
  Branches     2193     2193           
=======================================
  Hits        11439    11439           
  Misses        678      678           
  Partials      334      334           

Impacted Files	Coverage Δ
nibabel/viewers.py	96.14% <0.00%> (ø)

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[effigies]

Is there a case where fig.canvas.manager can be None?

I've installed the minimum matplotlib (1.5.3) in a dev environment, and matplotlib.pyplot.gcf().canvas.manager resolved fine. If this was being conservative to support older versions, then we can probably drop it. If there are configurations where it can actually be None, then we should keep it.

[larsoner]

I think if you instantiate the figure instance yourself (e.g., to embed in a Qt application of your own) instead of going through pyplot it's probably None. I haven't checked but there is probably some condition like this that causes it to be None.

[larsoner]

Verified that indeed fig.canvas.manager is None if you embed on your own. But then I noticed that this code is in a block where we directly create fig using pyplot, so indeed we should be good to remove the conditional (I hope/presume)!

[effigies]

LGTM, thanks!