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).

src/_pytest/config/__init__.py

@@ -169,7 +169,7 @@ def get_config(args=None, plugins=None):
     config = Config(
         pluginmanager,
         invocation_params=Config.InvocationParams(
-            args=args, plugins=plugins, dir=Path().resolve()
+            args=args or (), plugins=plugins, dir=Path().resolve()
         ),
     )
 
@@ -654,7 +654,7 @@ class Config:
 
         Contains the following read-only attributes:
 
-        * ``args``: list of command-line arguments as passed to ``pytest.main()``.
+        * ``args``: tuple of command-line arguments as passed to ``pytest.main()``.
         * ``plugins``: list of extra plugins, might be None.
         * ``dir``: directory where ``pytest.main()`` was invoked from.
     """
@@ -667,13 +667,13 @@ class Config:
 
         .. note::
 
-            Currently the environment variable PYTEST_ADDOPTS is also handled by
-            pytest implicitly, not being part of the invocation.
+            Note that the environment variable ``PYTEST_ADDOPTS`` and the ``addopts``
+            ini option are handled by pytest, not being included in the ``args`` attribute.
 
             Plugins accessing ``InvocationParams`` must be aware of that.
         """
 
-        args = attr.ib()
+        args = attr.ib(converter=tuple)
         plugins = attr.ib()
         dir = attr.ib(type=Path)
 
@@ -938,7 +938,6 @@ class Config:
         assert not hasattr(
             self, "args"
         ), "can only parse cmdline args at most once per Config object"
-        assert self.invocation_params.args == args
         self.hook.pytest_addhooks.call_historic(
             kwargs=dict(pluginmanager=self.pluginmanager)
         )