Make InvocationParams.args a tuple

This avoids mutating the original list to reflect on InvocationParams,
which is supposed to be an immutable snapshot of the state of pytest.main()
at the moment of invocation (see pytest-dev/pytest-xdist#478).

testing/test_config.py

@@ -7,6 +7,7 @@ import _pytest._code
 import pytest
 from _pytest.compat import importlib_metadata
 from _pytest.config import _iter_rewritable_modules
+from _pytest.config import Config
 from _pytest.config.exceptions import UsageError
 from _pytest.config.findpaths import determine_setup
 from _pytest.config.findpaths import get_common_ancestor
@@ -456,7 +457,7 @@ class TestConfigFromdictargs:
 
         config = Config.fromdictargs(option_dict, args)
         assert config.args == ["a", "b"]
-        assert config.invocation_params.args == args
+        assert config.invocation_params.args == tuple(args)
         assert config.option.verbose == 4
         assert config.option.capture == "no"
 
@@ -1235,7 +1236,7 @@ def test_invocation_args(testdir):
     call = calls[0]
     config = call.item.config
 
-    assert config.invocation_params.args == [p, "-v"]
+    assert config.invocation_params.args == (p, "-v")
     assert config.invocation_params.dir == Path(str(testdir.tmpdir))
 
     plugins = config.invocation_params.plugins
@@ -1243,6 +1244,10 @@ def test_invocation_args(testdir):
     assert plugins[0] is plugin
     assert type(plugins[1]).__name__ == "Collect"  # installed by testdir.inline_run()
 
+    # args cannot be None
+    with pytest.raises(TypeError):
+        Config.InvocationParams(args=None, plugins=None, dir=Path())
+
 
 @pytest.mark.parametrize(
     "plugin",