Hoist passing through sys.argv at the CLI layer

`flake8.main.cli.main()` is the primary entry point for the command-line
implementation of flake8 (invoked via `__main__` or `console_scripts`).
Therefore, it is reasonable for the entry point to be responsible for
obtaining command line arguments from `sys.argv` there.

Note that `sys.argv[1:]` is necessary in order to strip off the script
name.  Formerly, this was not needed in
`Application.parse_preliminary_options_and_args()`, which was using
`sys.argv[:]` because the result of the argument parsing was just for
determining additional configuration to be loaded.  Then, the *real* CLI
argument parsing was forwarding the original `None` value to
`argparse.ArgumentParser.parse_args()`, which internally was obtaining
arguments as `sys.argv[1:]`.

Additionally, the contract for various argument parsing methods to be
guaranteed a `List[str]`.

tests/integration/test_main.py

@@ -126,3 +126,13 @@ def test_bug_report_successful(capsys):
     out, err = capsys.readouterr()
     assert json.loads(out)
     assert err == ''
+
+
+def test_obtaining_args_from_sys_argv_when_not_explicity_provided(capsys):
+    """Test that arguments are obtained from 'sys.argv'."""
+    with mock.patch('sys.argv', ['flake8', '--help']):
+        _call_main(None)
+
+    out, err = capsys.readouterr()
+    assert out.startswith('usage: flake8 [options] file file ...\n')
+    assert err == ''