Convert Python type hinting to be PEP-585 Compliant (#4083)

Python [PEP-585](https://peps.python.org/pep-0585/) replaces a number of
`typing` module types with built-in equivalents and `collections.abc`
versions as of Python 3.9, with the aim of eventually removing the
`typing` module versions of these classes altogether. Since the minimum
required version of Python listed in the [Contribution Tools
document](https://github.com/carbon-language/carbon-lang/blob/trunk/docs/project/contribution_tools.md#main-tools)
is 3.9, the type hints in the various python files in the repo can be
updated to this style of type hint without a need for backwards
compatibility.

Feel free to close if this isn't a desired change at this time!

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

scripts/source_stats.py

@@ -16,7 +16,7 @@ from multiprocessing import Pool
 import re
 import termplotlib as tpl  # type:ignore
 from pathlib import Path
-from typing import Dict, List, Optional
+from typing import Optional
 from dataclasses import dataclass, field, asdict
 from collections import Counter
 
@@ -167,7 +167,7 @@ def scan_file(file: Path) -> Stats:
     return stats
 
 
-def parse_args(args: Optional[List[str]] = None) -> argparse.Namespace:
+def parse_args(args: Optional[list[str]] = None) -> argparse.Namespace:
     """Parsers command-line arguments and flags."""
     parser = argparse.ArgumentParser(description=__doc__)
     parser.add_argument(
@@ -232,7 +232,7 @@ Fraction IDs: {stats.identifiers / tokens}
     )
 
     def print_histogram(
-        title: str, data: Dict[int, int], column_format: str
+        title: str, data: dict[int, int], column_format: str
     ) -> None:
         print()
         key_min = min(data.keys())