Use the standard library's enterContext

It looks like it was added in python 3.11, so there isn't much reason to
use absl's enter_context anymore.

rock_paper_sand/config_test.py

@@ -35,7 +35,7 @@
 
 class ConfigTest(parameterized.TestCase):
     def _config_with_null_session(self) -> config.Config:
-        session = self.enter_context(network.null_requests_session())
+        session = self.enterContext(network.null_requests_session())
         return config.Config.from_config_file(
             wikidata_session=session,
             justwatch_session=session,
@@ -77,7 +77,7 @@ def test_invalid_config(
         error_regex: str,
         error_notes: Sequence[str],
     ) -> None:
-        self.enter_context(
+        self.enterContext(
             flagsaver.flagsaver(
                 (
                     flags_and_constants.CONFIG_FILE,
@@ -227,7 +227,7 @@ def test_lint(
         config_data: Any,
         expected_results: Any,
     ) -> None:
-        self.enter_context(
+        self.enterContext(
             flagsaver.flagsaver(
                 (
                     flags_and_constants.CONFIG_FILE,

rock_paper_sand/state_test.py

@@ -33,7 +33,7 @@ def setUp(self) -> None:
             / "new-dir"
             / "new-file.binpb"
         )
-        self.enter_context(
+        self.enterContext(
             flagsaver.flagsaver(
                 (flags_and_constants.STATE_FILE, str(self._state_path))
             )

rock_paper_sand/subcommand_test.py

@@ -56,7 +56,7 @@ def test_container_runs_subcommand(self) -> None:
 
     def test_container_prints_help(self) -> None:
         parser = argparse.ArgumentParser()
-        mock_print_help = self.enter_context(
+        mock_print_help = self.enterContext(
             mock.patch.object(
                 parser, "print_help", autospec=True, spec_set=True
             )