Fix build errors

[davidag]

Two problems have been found in the build:

1. Because of an incompatibility of python-sugar with pytest 5.4.0 and 5.4.1 this changes are required for the tests to run correctly until a new version of pytest solves it.

View full error message

> Run poetry run invoke tests
INTERNALERROR> Traceback (most recent call last):
INTERNALERROR>   File "/home/runner/.cache/pypoetry/virtualenvs/pelican-webring-yOIIE9ev-py3.7/lib/python3.7/site-packages/_pytest/main.py", line 187, in wrap_session
INTERNALERROR>     config._do_configure()
INTERNALERROR>   File "/home/runner/.cache/pypoetry/virtualenvs/pelican-webring-yOIIE9ev-py3.7/lib/python3.7/site-packages/_pytest/config/__init__.py", line 820, in _do_configure
INTERNALERROR>     self.hook.pytest_configure.call_historic(kwargs=dict(config=self))
INTERNALERROR>   File "/home/runner/.cache/pypoetry/virtualenvs/pelican-webring-yOIIE9ev-py3.7/lib/python3.7/site-packages/pluggy/hooks.py", line 308, in call_historic
INTERNALERROR>     res = self._hookexec(self, self.get_hookimpls(), kwargs)
INTERNALERROR>   File "/home/runner/.cache/pypoetry/virtualenvs/pelican-webring-yOIIE9ev-py3.7/lib/python3.7/site-packages/pluggy/manager.py", line 93, in _hookexec
INTERNALERROR>     return self._inner_hookexec(hook, methods, kwargs)
INTERNALERROR>   File "/home/runner/.cache/pypoetry/virtualenvs/pelican-webring-yOIIE9ev-py3.7/lib/python3.7/site-packages/pluggy/manager.py", line 87, in <lambda>
INTERNALERROR>     firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
INTERNALERROR>   File "/home/runner/.cache/pypoetry/virtualenvs/pelican-webring-yOIIE9ev-py3.7/lib/python3.7/site-packages/pluggy/callers.py", line 208, in _multicall
INTERNALERROR>     return outcome.get_result()
INTERNALERROR>   File "/home/runner/.cache/pypoetry/virtualenvs/pelican-webring-yOIIE9ev-py3.7/lib/python3.7/site-packages/pluggy/callers.py", line 80, in get_result
INTERNALERROR>     raise ex[1].with_traceback(ex[2])
INTERNALERROR>   File "/home/runner/.cache/pypoetry/virtualenvs/pelican-webring-yOIIE9ev-py3.7/lib/python3.7/site-packages/pluggy/callers.py", line 187, in _multicall
INTERNALERROR>     res = hook_impl.function(*args)
INTERNALERROR>   File "/home/runner/.cache/pypoetry/virtualenvs/pelican-webring-yOIIE9ev-py3.7/lib/python3.7/site-packages/pytest_sugar.py", line 176, in pytest_configure
INTERNALERROR>     sugar_reporter = SugarTerminalReporter(standard_reporter)
INTERNALERROR>   File "/home/runner/.cache/pypoetry/virtualenvs/pelican-webring-yOIIE9ev-py3.7/lib/python3.7/site-packages/pytest_sugar.py", line 214, in __init__
INTERNALERROR>     self.writer = self._tw
INTERNALERROR> AttributeError: can't set attribute
##[error]Process completed with exit code 3.

2. The Pelican dev version was not being used, which caused an import error when running tests depending on the module pelican.tests:

View full error message

ImportError while importing test module '/home/runner/work/webring/webring/pelican/plugins/webring/test_webring.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
pelican/plugins/webring/test_webring.py:14: in <module>
    from pelican.tests.support import get_context, get_settings, module_exists
E   ModuleNotFoundError: No module named 'pelican.tests'
!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!