[MAINT] add code coverage

[Remi-Gau]

mostly to see if there are any obvious holes in what is being tested

[Remi-Gau]

remove this skipif because we never test with python less than 3.6

[Remi-Gau]

coverage is printed in CI:

https://github.com/sphinx-contrib/matlabdomain/actions/runs/18255487017/job/51976173683?pr=297#step:4:238

Name                                                                                      Stmts   Miss Branch BrPart  Cover
---------------------------------------------------------------------------------------------------------------------------
.tox/3.11-sphinx70/lib/python3.11/site-packages/sphinxcontrib/mat_directives.py              35      6      4      1    82%
.tox/3.11-sphinx70/lib/python3.11/site-packages/sphinxcontrib/mat_documenters.py            808    167    452     67    75%
.tox/3.11-sphinx70/lib/python3.11/site-packages/sphinxcontrib/mat_lexer.py                   12      0      0      0   100%
.tox/3.11-sphinx70/lib/python3.11/site-packages/sphinxcontrib/mat_tree_sitter_parser.py     395     59    228     25    78%
.tox/3.11-sphinx70/lib/python3.11/site-packages/sphinxcontrib/mat_types.py                  544     40    192     19    91%
.tox/3.11-sphinx70/lib/python3.11/site-packages/sphinxcontrib/matlab.py                     493    107    216     42    74%
tests/helper.py                                                                              10      3      2      1    67%
tests/roots/test_autodoc/conf.py                                                              8      0      0      0   100%
tests/roots/test_classfolder/conf.py                                                          9      0      0      0   100%
tests/roots/test_duplicate_link/conf.py                                                      19      0      0      0   100%
tests/roots/test_module_class_names/conf.py                                                  11      0      0      0   100%
tests/roots/test_no_matlab_src_dir/conf.py                                                    6      0      0      0   100%
tests/roots/test_numad/conf.py                                                                9      0      0      0   100%
tests/roots/test_package_links/conf.py                                                        8      0      0      0   100%
tests/roots/test_package_prefix/conf.py                                                       8      0      0      0   100%
tests/roots/test_pymat/conf.py                                                                8      0      0      0   100%
tests/roots/test_pymat/pysrc/func.py                                                          2      1      0      0    50%
tests/roots/test_pymat_common_root/base/PythonClass.py                                        3      1      0      0    67%
tests/roots/test_pymat_common_root/base/__init__.py                                           0      0      0      0   100%
tests/roots/test_pymat_common_root/conf.py                                                    8      0      0      0   100%
tests/roots/test_skipping_module_members/conf.py                                             19      0      0      0   100%
tests/test_autodoc.py                                                                       167      1      2      1    99%
tests/test_autodoc_properties.py                                                             23      1      6      1    93%
tests/test_autodoc_short_links.py                                                           156      1      2      1    99%
tests/test_classfolder.py                                                                    25      0      0      0   100%
tests/test_classfolder_class_name.py                                                         16      0      0      0   100%
tests/test_docs/conf.py                                                                      14      0      0      0   100%
tests/test_duplicated_link.py                                                                29      1      2      1    94%
tests/test_lexer.py                                                                          11      0      0      0   100%
tests/test_matlabify.py                                                                     157      1      2      1    99%
tests/test_module_class_names.py                                                             22      1      2      1    92%
tests/test_no_matlab_src_dir.py                                                              16      1      2      1    89%
tests/test_numad.py                                                                          21      1      2      1    91%
tests/test_package_links.py                                                                  23      1      2      1    92%
tests/test_package_prefix.py                                                                 25      1      2      1    93%
tests/test_parse_mfile.py                                                                   533      1      2      1    99%
tests/test_parse_mlappfile.py                                                                13      1      2      1    87%
tests/test_pymat.py                                                                          19      1      2      1    90%
tests/test_pymat_common_root.py                                                              19      1      2      1    90%
tests/test_shortest_name.py                                                                  38      0      0      0   100%
tests/test_skipping_module_members.py                                                        17      1      2      1    89%
---------------------------------------------------------------------------------------------------------------------------
TOTAL                                                                                      3759    399   1128    170    85%

but could be uploaded to codecov