Fail CI on encountering deprecation warnings (#404)

Because we run flake8 via a subprocess in our test suite, pytest can't
detect any DeprecationWarnings emitted while the plugin is being run.
Instead, use Python's warnings filter to have all warnings encountered
in the subprocess be printed to stderr. This will cause tests to fail with
a nice diff if any deprecation warnings are encountered in the
subprocess.

Closes #392

pyproject.toml

@@ -102,3 +102,4 @@ ignore_missing_imports = true
 
 [tool.pytest.ini_options]
 addopts = "--doctest-modules"
+filterwarnings = ["error"]

tests/test_pyi_files.py

@@ -42,23 +42,42 @@ def test_pyi_file(path: str) -> None:
         option = flag.split("=")[0]
         assert option != "--ignore", bad_flag_msg
 
+    # Silence DeprecationWarnings from our dependencies (pyflakes, flake8-bugbear, etc.)
+    #
+    # For DeprecationWarnings coming from flake8-pyi itself,
+    # print the first occurence of each warning to stderr.
+    # This will fail CI the same as `-Werror:::pyi`,
+    # but the test failure report that pytest gives is much easier to read
+    # if we use `-Wdefault:::pyi`
+    flake8_invocation = [
+        sys.executable,
+        "-Wignore",
+        "-Wdefault:::pyi",
+        "-m",
+        "flake8",
+        "-j0",
+    ]
+
     run_results = [
         # Passing a file on command line
         subprocess.run(
-            ["flake8", "-j0", *flags, path],
+            [*flake8_invocation, *flags, path],
             env={**os.environ, "PYTHONPATH": "."},
-            stdout=subprocess.PIPE,
+            capture_output=True,
+            text=True,
         ),
         # Passing "-" as the file, and reading from stdin instead
         subprocess.run(
-            ["flake8", "-j0", "--stdin-display-name", path, *flags, "-"],
+            [*flake8_invocation, "--stdin-display-name", path, *flags, "-"],
             env={**os.environ, "PYTHONPATH": "."},
-            input=file_contents.encode("utf-8"),
-            stdout=subprocess.PIPE,
+            input=file_contents,
+            capture_output=True,
+            text=True,
         ),
     ]
 
     for run_result in run_results:
-        output = run_result.stdout.decode("utf-8")
-        output = re.sub(":[0-9]+: ", ": ", output)  # ignore column numbers
+        output = re.sub(":[0-9]+: ", ": ", run_result.stdout)  # ignore column numbers
+        if run_result.stderr:
+            output += "\n" + run_result.stderr
         assert output == expected_output