Merge branch 'extend_exclude_is_files' into 'master'

treat --extend-exclude as a file list

Closes #653

See merge request pycqa/flake8!432

src/flake8/main/options.py

@@ -156,6 +156,7 @@ def register_default_options(option_manager):
         default="",
         parse_from_config=True,
         comma_separated_list=True,
+        normalize_paths=True,
         help="Comma-separated list of files or directories to add to the list"
         " of excluded ones.",
     )

tests/integration/test_main.py

@@ -114,7 +114,7 @@ def test_extend_exclude(tmpdir, capsys):
         tmpdir.mkdir(d).join('t.py').write('import os\nimport sys\n')
 
     with tmpdir.as_cwd():
-        _call_main(['--extend-exclude=vendor,legacy'], retv=1)
+        _call_main(['--extend-exclude=vendor,legacy/'], retv=1)
 
     out, err = capsys.readouterr()
     expected_out = '''\