Remove more Python 3.8 cruft (#13787)

tests/check_typeshed_structure.py

@@ -9,7 +9,6 @@ from __future__ import annotations
 
 import os
 import re
-import sys
 from pathlib import Path
 
 from ts_utils.metadata import read_metadata
@@ -170,7 +169,6 @@ def check_requirement_pins() -> None:
 
 
 if __name__ == "__main__":
-    assert sys.version_info >= (3, 9), "Python 3.9+ is required to run this test"
     check_versions_file()
     check_metadata()
     check_requirement_pins()

tests/mypy_test.py

@@ -393,8 +393,7 @@ def stdlib_module_name_from_path(path: Path) -> str:
     assert path.suffix == ".pyi"
     parts = list(path.parts[1:-1])
     if path.parts[-1] != "__init__.pyi":
-        # TODO: Python 3.9+: Use removesuffix.
-        parts.append(path.parts[-1][:-4])
+        parts.append(path.parts[-1].removesuffix(".pyi"))
     return ".".join(parts)