Keep sugar for local use, but use instafail on CI

There are two benefits of the pytest-sugar plugin:

1. Pretty output.
2. Show details on each failure immediately instead of at the end.

The first benefit is effectively local-only, because extra newlines
are appearing when it runs on CI, both with and without -v.

The second benefit applies both locally and on CI.

So this adds the pytest-instafail plugin and uses it on CI to get
the second benefit. It is not set up to run automatically, and
pytest-sugar still is (though no longer forced), so local testing
retains no benefit and we don't have a clash.

The name "instafail" refers only to instantly *seeing* failures:
it does not cause the pytest runner to stop earlier than otherwise.

.github/workflows/cygwin-test.yml

@@ -64,4 +64,4 @@ jobs:
     - name: Test with pytest
       run: |
         set +x
-        /usr/bin/python -m pytest -v
+        /usr/bin/python -m pytest -p no:sugar -v --instafail

.github/workflows/pythonpackage.yml

@@ -78,7 +78,7 @@ jobs:
 
     - name: Test with pytest
       run: |
-        pytest -v
+        pytest -v -p no:sugar --instafail
       continue-on-error: false
 
     - name: Documentation

pyproject.toml

@@ -5,7 +5,7 @@ build-backend = "setuptools.build_meta"
 [tool.pytest.ini_options]
 python_files = 'test_*.py'
 testpaths = 'test'  # space separated list of paths from root e.g test tests doc/testing
-addopts = '--cov=git --cov-report=term --force-sugar --disable-warnings'
+addopts = '--cov=git --cov-report=term --disable-warnings'
 filterwarnings = 'ignore::DeprecationWarning'
 # --cov   coverage
 # --cov-report term  # send report to terminal term-missing -> terminal with line numbers  html  xml

test-requirements.txt

@@ -5,4 +5,5 @@ mypy
 pre-commit
 pytest
 pytest-cov
+pytest-instafail
 pytest-sugar