Fix running test_internal_cpp_api directly.

QuLogic · QuLogic · commit b672f68f89fb · 2020-05-07T23:41:17.000-04:00
The `matplotlib._tri` module is lazily loaded, so using it directly (as
done in this test) may fail if something else doesn't load it. This test
would fail if it was the first tri-related test run in a process (if
testing in parallel), or if just called as the only test, i.e., this
commit fixes:
```
$ pytest -k test_internal_cpp_api lib/matplotlib/tests/test_triangulation.py
__________ test_internal_cpp_api __________

    def test_internal_cpp_api():
        # Following github issue 8197.

        # C++ Triangulation.
        with pytest.raises(
                TypeError,
                match=r'function takes exactly 7 arguments $0 given$'):
&gt;           mpl._tri.Triangulation()
E           AttributeError: module 'matplotlib' has no attribute '_tri'

lib/matplotlib/tests/test_triangulation.py:1039: AttributeError
```
diff --git a/lib/matplotlib/tests/test_triangulation.py b/lib/matplotlib/tests/test_triangulation.py
@@ -1031,6 +1031,7 @@ def test_tricontourf_decreasing_levels():
 
 def test_internal_cpp_api():
     # Following github issue 8197.
+    from matplotlib import _tri  # noqa: ensure lazy-loaded module *is* loaded.
 
     # C++ Triangulation.
     with pytest.raises(
