Enable pytest openfiles checking

Disable .ttf files opened by matplotlib.

.github/workflows/build.yaml

@@ -37,7 +37,7 @@ jobs:
         run: pip install -v .
 
       - name: Run tests
-        run: pytest -r a -v -n 3
+        run: pytest -r a -v -n 3 --open-files
 
       - name: Install documenteer
         run: pip install documenteer[pipelines]

setup.cfg

@@ -28,6 +28,7 @@ tests_require =
   pytest >= 3.2
   flake8 >= 3.7.5
   pytest-flake8 >= 1.0.4
+  pytest-openfiles >= 0.5.0
   numpy >= 1.17
   matplotlib >= 3.0.3
   pyarrow >= 0.16
@@ -54,3 +55,5 @@ exclude = __init__.py
 [tool:pytest]
 addopts = --flake8
 flake8-ignore = E133 E226 E228 N802 N803 N806 N812 N815 N816 W503
+# The matplotlib test may not release font files
+open_files_ignore = "*.ttf"