enforce agg backend during testing (windows)

[marscher]

Changes

As the TK backend of matplotlib sometimes shows strange behavior on Windows, we want to use a different backend (Agg).

The exception thrown by the faulty tk initialization looks like this (for the archives);

     fig, ax = plt.subplots()
c:\hostedtoolcache\windows\python\3.9.2\x64\lib\site-packages\matplotlib\_api\deprecation.py:471: in wrapper
    return func(*args, **kwargs)
c:\hostedtoolcache\windows\python\3.9.2\x64\lib\site-packages\matplotlib\pyplot.py:1410: in subplots
    fig = figure(**fig_kw)
c:\hostedtoolcache\windows\python\3.9.2\x64\lib\site-packages\matplotlib\pyplot.py:768: in figure
    manager = new_figure_manager(
c:\hostedtoolcache\windows\python\3.9.2\x64\lib\site-packages\matplotlib\pyplot.py:316: in new_figure_manager
    return _backend_mod.new_figure_manager(*args, **kwargs)
c:\hostedtoolcache\windows\python\3.9.2\x64\lib\site-packages\matplotlib\backend_bases.py:3545: in new_figure_manager
    return cls.new_figure_manager_given_figure(num, fig)
c:\hostedtoolcache\windows\python\3.9.2\x64\lib\site-packages\matplotlib\backends\_backend_tk.py:870: in new_figure_manager_given_figure
    window = tk.Tk(className="matplotlib")
c:\hostedtoolcache\windows\python\3.9.2\x64\lib\tkinter\__init__.py:2270: in __init__
    self.tk = _tkinter.create(screenName, baseName, className, interactive, wantobjects, useTk, sync, use)
E   _tkinter.TclError: Can't find a usable init.tcl in the following directories: 
E       {c:\hostedtoolcache\windows\python\3.9.2\x64\tcl\tcl8.6}
E   
E   c:/hostedtoolcache/windows/python/3.9.2/x64/tcl/tcl8.6/init.tcl: couldn't read file "c:/hostedtoolcache/windows/python/3.9.2/x64/tcl/tcl8.6/init.tcl": No error
E   couldn't read file "c:/hostedtoolcache/windows/python/3.9.2/x64/tcl/tcl8.6/init.tcl": No error
E       while executing
E   "source c:/hostedtoolcache/windows/python/3.9.2/x64/tcl/tcl8.6/init.tcl"
E       ("uplevel" body line 1)
E       invoked from within
E   "uplevel #0 [list source $tclfile]"
E   
E   
E   This probably means that Tcl wasn't installed properly.

Related Issues

Closes #321

[CagtayFabry]

maybe we could circumvent this by using https://stackoverflow.com/a/60127793 ?

[codecov]

Codecov Report

Merging #322 (10eb512) into master (1c7bf79) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #322   +/-   ##
=======================================
  Coverage   96.86%   96.86%           
=======================================
  Files          82       82           
  Lines        4727     4727           
=======================================
  Hits         4579     4579           
  Misses        148      148           

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[CagtayFabry]

my review will consist of a few reruns of the tests here 😁 😉

[CagtayFabry]

looking good 🚀