Lower the scope of simple_script fixture to apply isolate to it

Lower the scope of `simple_script` fixture used in `test_inspect`
from `session` to (implicit) `function`, to make it match the `isolate`
fixture and therefore make the fixture run in isolation.
Per the documentation [1], pytest initializes higher-scope fixtures
first, and lower-scope fixtures cannot be used in them.  Since within
the same scope, autouse fixtures are always loaded before regular
fixtures, no further adjustment is necessary.  This should have no ill
side effects, as the fixture is used by a single test only.

[1] https://docs.pytest.org/en/stable/reference/fixtures.html#fixture-instantiation-order

Author: mgorny

tests/functional/test_inspect.py

@@ -5,7 +5,7 @@
 from tests.lib import PipTestEnvironment, ScriptFactory, TestData
 
 
-@pytest.fixture(scope="session")
+@pytest.fixture
 def simple_script(
     tmpdir_factory: pytest.TempPathFactory,
     script_factory: ScriptFactory,