Add initial compatibility test.

The compatibility test is currently failing from what looks like an
error with flake8. (It is unable to process files in a nested directory
when `--enable-extensions=TRUE` is passed to it.)

Also, add poetry config.

integration_tests/compatibility.py

@@ -0,0 +1,33 @@
+"""Tests for compatibility with other flake8 plugins."""
+
+import subprocess
+from unittest import TestCase
+
+# Regression introduced by flake8 in 3.3.0
+# Bad:  3.3.0
+
+class CompatibilityTest(TestCase):
+
+    def test_flake8_docstrings(self):
+        proc = subprocess.run([
+                'flake8',
+                '--isolated',
+                '--enable-extensions=TRUE',
+                'integration_tests/files/example-ascii.py',
+            ], stdout=subprocess.PIPE
+        )
+        result = proc.stdout.decode('utf8')
+        print(result)
+
+        self.assertTrue(
+            'D401' in result,
+            'The file should contain an error from flake8-docstrings.',
+        )
+        self.assertTrue(
+            'RST399' in result,
+            'The file should contain an error from flake8-rst-docstrings',
+        )
+        self.assertTrue(
+            'DAR101' in result,
+            'The file should contain an error from darglint.',
+        )

integration_tests/files/problematic.py

@@ -0,0 +1,16 @@
+
+def problematic_function(arg1):
+    """Returns double the value of the first argument.
+
+    Args:
+        a: The first argument.
+
+    Returns:
+        Two times the value of the first argument.
+
+    """
+    return arg1 * 2
+
+
+def problematic_rst():
+    """"""