Fix for matplotlib 3.10.dev and other tidy up

[ericpre]

Similar as hyperspy/hyperspy#3367.

Other tidying up:

• Use qt instead of qt4 when setting ETSConfig.toolkit.
• Remove empty conftest.py file.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 0.00%. Comparing base (9e90eef) to head (42aa4d7).

Files	Patch %	Lines
hyperspy_gui_traitsui/__init__.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #78       +/-   ##
==========================================
- Coverage   44.83%   0.00%   -44.84%     
==========================================
  Files          12      10        -2     
  Lines         707     446      -261     
  Branches      104       0      -104     
==========================================
- Hits          317       0      -317     
- Misses        378     446       +68     
+ Partials       12       0       -12     

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