GHA: Set mpl backend

[dweindl]

Avoid such issues:

    fig = plt.figure()
.tox\windows\Lib\site-packages\matplotlib\pyplot.py:934: in figure
    manager = new_figure_manager(
.tox\windows\Lib\site-packages\matplotlib\pyplot.py:465: in new_figure_manager
    return _get_backend_mod().new_figure_manager(*args, **kwargs)
.tox\windows\Lib\site-packages\matplotlib\backend_bases.py:3402: in new_figure_manager
    return cls.new_figure_manager_given_figure(num, fig)
.tox\windows\Lib\site-packages\matplotlib\backend_bases.py:3407: in new_figure_manager_given_figure
    return cls.FigureCanvas.new_manager(figure, num)
.tox\windows\Lib\site-packages\matplotlib\backend_bases.py:1785: in new_manager
    return cls.manager_class.create_with_canvas(cls, figure, num)
.tox\windows\Lib\site-packages\matplotlib\backends\_backend_tk.py:486: in create_with_canvas
    window = tk.Tk(className="matplotlib")
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <tkinter.Tk object .>, screenName = None, baseName = 'pytest.EXE'
className = 'matplotlib', useTk = True, sync = False, use = None

    def __init__(self, screenName=None, baseName=None, className='Tk',
                 useTk=True, sync=False, use=None):
        """Return a new top level widget on screen SCREENNAME. A new Tcl interpreter will
        be created. BASENAME will be used for the identification of the profile file (see
        readprofile).
        It is constructed from sys.argv[0] without extensions if None is given. CLASSNAME
        is the name of the widget class."""
        self.master = None
        self.children = {}
        self._tkloaded = False
        # to avoid recursions in the getattr code in case of failure, we
        # ensure that self.tk is always _something_.
        self.tk = None
        if baseName is None:
            import os
            baseName = os.path.basename(sys.argv[0])
            baseName, ext = os.path.splitext(baseName)
            if ext not in ('.py', '.pyc'):
                baseName = baseName + ext
        interactive = False
>       self.tk = _tkinter.create(screenName, baseName, className, interactive, wantobjects, useTk, sync, use)
E       _tkinter.TclError: Can't find a usable tk.tcl in the following directories: 
E           C:/hostedtoolcache/windows/Python/3.11.7/x64/tcl/tcl8.6/tk8.6 C:/hostedtoolcache/windows/Python/3.11.7/x64/tcl/tk8.6 C:/hostedtoolcache/windows/Python/3.11.7/lib/tk8.6 C:/hostedtoolcache/windows/Python/3.11.7/lib/tk8.6 C:/hostedtoolcache/windows/Python/lib/tk8.6 C:/hostedtoolcache/windows/Python/3.11.7/library
E       
E       C:/hostedtoolcache/windows/Python/3.11.7/x64/tcl/tk8.6/tk.tcl: couldn't read file "C:/hostedtoolcache/windows/Python/3.11.7/x64/tcl/tk8.6/ttk/scrollbar.tcl": no such file or directory
E       couldn't read file "C:/hostedtoolcache/windows/Python/3.11.7/x64/tcl/tk8.6/ttk/scrollbar.tcl": no such file or directory
E           while executing
E       "source -encoding utf-8 [file join $::ttk::library scrollbar.tcl]"
E           (file "C:/hostedtoolcache/windows/Python/3.11.7/x64/tcl/tk8.6/ttk/ttk.tcl" line 102)
E           invoked from within
E       "source -encoding utf-8 C:/hostedtoolcache/windows/Python/3.11.7/x64/tcl/tk8.6/ttk/ttk.tcl"
E           ("uplevel" body line 1)
E           invoked from within
E       "uplevel \#0 [list source -encoding utf-8 $::ttk::library/ttk.tcl]"
E           (file "C:/hostedtoolcache/windows/Python/3.11.7/x64/tcl/tk8.6/tk.tcl" line 706)
E           invoked from within
E       "source C:/hostedtoolcache/windows/Python/3.11.7/x64/tcl/tk8.6/tk.tcl"
E           ("uplevel" body line 1)
E           invoked from within
E       "uplevel #0 [list source $file]"
E       
E       
E       This probably means that tk wasn't installed properly.

C:\hostedtoolcache\windows\Python\3.11.7\x64\Lib\tkinter\__init__.py:2326: TclError

e.g. here https://github.com/ICB-DCM/pyPESTO/actions/runs/7261863194/job/19783894242?pr=1243

[codecov-commenter]

Codecov Report

Attention: 853 lines in your changes are missing coverage. Please review.

Comparison is base (160c2a8) 88.16% compared to head (36b5630) 46.54%.
Report is 483 commits behind head on develop.

Files	Patch %	Lines
pypesto/ensemble/ensemble.py	12.88%	196 Missing ⚠️
pypesto/history/hdf5.py	34.33%	174 Missing ⚠️
pypesto/ensemble/util.py	15.59%	92 Missing ⚠️
pypesto/history/csv.py	30.70%	79 Missing ⚠️
...ypesto/hierarchical/spline_approximation/solver.py	82.62%	53 Missing ⚠️
pypesto/history/base.py	76.66%	49 Missing ⚠️
pypesto/hierarchical/optimal_scaling/solver.py	93.53%	26 Missing ⚠️
pypesto/hierarchical/petab.py	84.76%	23 Missing ⚠️
pypesto/engine/mpi_pool.py	0.00%	22 Missing ⚠️
pypesto/hierarchical/problem.py	87.23%	18 Missing ⚠️
... and 17 more

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Additional details and impacted files

@@             Coverage Diff              @@
##           develop    #1259       +/-   ##
============================================
- Coverage    88.16%   46.54%   -41.62%     
============================================
  Files           79      148       +69     
  Lines         5257    12067     +6810     
============================================
+ Hits          4635     5617      +982     
- Misses         622     6450     +5828     

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[Doresic]

This is great, ty!