pdf generation fails with nbconvert >= 6.0

[...]
.  build_book
INFO:bookbook.latex:Combined 4 files
INFO:bookbook.latex:source_dir.exists()=True Path(template_file).exists()=True
INFO:bookbook.latex:Converting to pdf
INFO:bookbook.latex:Path(exporter.template_file).exists()=True
TaskError - taskid:build_book
PythonAction Error
Traceback (most recent call last):
  File "/Users/boileau/Documents/Enseignement/2020_Calcul_parallele/cours-opencl/.env/lib/python3.8/site-packages/doit/action.py", line 437, in execute
    returned_value = self.py_callable(*self.args, **kwargs)
  File "/Users/boileau/Documents/Enseignement/2020_Calcul_parallele/cours-opencl/.env/lib/python3.8/site-packages/nbcourse/nbcourse.py", line 210, in build_book
    book.build()
  File "/Users/boileau/Documents/Enseignement/2020_Calcul_parallele/cours-opencl/.env/lib/python3.8/site-packages/nbcourse/book.py", line 64, in build
    combine_and_convert(Path('.'),
  File "/Users/boileau/Documents/Enseignement/2020_Calcul_parallele/cours-opencl/.env/lib/python3.8/site-packages/bookbook/latex.py", line 147, in combine_and_convert
    export(combined_nb, output_file, pdf=pdf, template_file=template_file)
  File "/Users/boileau/Documents/Enseignement/2020_Calcul_parallele/cours-opencl/.env/lib/python3.8/site-packages/bookbook/latex.py", line 140, in export
    output, resources = exporter.from_notebook_node(combined_nb, resources)
  File "/Users/boileau/Documents/Enseignement/2020_Calcul_parallele/cours-opencl/.env/lib/python3.8/site-packages/nbconvert/exporters/pdf.py", line 168, in from_notebook_node
    latex, resources = super().from_notebook_node(
  File "/Users/boileau/Documents/Enseignement/2020_Calcul_parallele/cours-opencl/.env/lib/python3.8/site-packages/nbconvert/exporters/latex.py", line 77, in from_notebook_node
    return super().from_notebook_node(nb, resources, **kw)
  File "/Users/boileau/Documents/Enseignement/2020_Calcul_parallele/cours-opencl/.env/lib/python3.8/site-packages/nbconvert/exporters/templateexporter.py", line 384, in from_notebook_node
    output = self.template.render(nb=nb_copy, resources=resources)
  File "/Users/boileau/Documents/Enseignement/2020_Calcul_parallele/cours-opencl/.env/lib/python3.8/site-packages/nbconvert/exporters/templateexporter.py", line 148, in template
    self._template_cached = self._load_template()
  File "/Users/boileau/Documents/Enseignement/2020_Calcul_parallele/cours-opencl/.env/lib/python3.8/site-packages/nbconvert/exporters/templateexporter.py", line 355, in _load_template
    return self.environment.get_template(template_file)
  File "/Users/boileau/Documents/Enseignement/2020_Calcul_parallele/cours-opencl/.env/lib/python3.8/site-packages/jinja2/environment.py", line 888, in get_template
    return self._load_template(name, self.make_globals(globals))
  File "/Users/boileau/Documents/Enseignement/2020_Calcul_parallele/cours-opencl/.env/lib/python3.8/site-packages/jinja2/environment.py", line 861, in _load_template
    template = self.loader.load(self, name, globals)
  File "/Users/boileau/Documents/Enseignement/2020_Calcul_parallele/cours-opencl/.env/lib/python3.8/site-packages/jinja2/loaders.py", line 437, in load
    raise TemplateNotFound(name)
jinja2.exceptions.TemplateNotFound: bookbook_filename.tex

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