Add test for generating api docs

[sverhoeven]

Instead of make html using sphinx-build ... for better cross-platform compatibility.

Also got tests to work on Windows CI (refs #223). It also still works on my laptop:

PS C:\Users\stefanv\git\NLeSC\python-template> .\env\Scripts\pytest.exe -v
==================================================== test session starts ====================================================
platform win32 -- Python 3.9.4, pytest-4.6.11, py-1.10.0, pluggy-0.13.1 -- c:\users\stefanv\git\nlesc\python-template\env\scripts\python.exe
cachedir: .pytest_cache
rootdir: C:\Users\stefanv\git\NLeSC\python-template, inifile: setup.cfg, testpaths: tests
plugins: cookies-0.5.1, cov-2.11.1
collected 8 items

tests/test_project.py::test_project_folder PASSED                                                                      [ 12%]
tests/test_project.py::test_pytest PASSED                                                                              [ 25%]
tests/test_project.py::test_subpackage PASSED                                                                          [ 37%]
tests/test_project.py::test_generate_api_docs PASSED                                                                   [ 50%]

Refs #213

[fdiblen]

nice! 👍

[jspaaks]

my goodness what an absolute disaster windows is. I was not able to run the tests from the template level, but at least I could generate the package and then run pytest and the sphinx build directly.

For the longer term, I think we should consider having two workflows, one to test the template, the other to test the generated package. The latter workflow would then go through the steps of creating a virtual env, install deps, etc, in the workflow file, not as part of the test.

[sverhoeven]

Thanks for reviewing. I am just happy it works on CI now.