Make test scripts pass mypy --strict (#7745)

- Add several type hints to untyped functions.
- While we're at it, upgrade several annotations to use modern syntax by using `from __future__ import annotations`. This means that the tests can't be run on Python 3.6, but 3.6 is EOL, and it is already the case that some scripts in this directory can only be run on more recent Python versions. E.g. `check_new_syntax.py` uses `ast.unparse`, which is only available in Python 3.9+.
- Fix a few pieces of code that didn't type check.

tests/mypy_test.py

@@ -9,6 +9,7 @@ Approach:
 2. Compute appropriate arguments for mypy
 3. Pass those arguments to mypy.api.run()
 """
+from __future__ import annotations
 
 import argparse
 import os
@@ -16,7 +17,7 @@ import re
 import sys
 import tempfile
 from pathlib import Path
-from typing import TYPE_CHECKING, Dict, List, NamedTuple, Set, Tuple
+from typing import TYPE_CHECKING, NamedTuple
 
 if TYPE_CHECKING:
     from _typeshed import StrPath
@@ -61,10 +62,11 @@ def match(fn: str, args: argparse.Namespace) -> bool:
 
 
 _VERSION_LINE_RE = re.compile(r"^([a-zA-Z_][a-zA-Z0-9_.]*): ([23]\.\d{1,2})-([23]\.\d{1,2})?$")
-VersionsTuple: TypeAlias = Tuple[int, int]
+MinVersion: TypeAlias = tuple[int, int]
+MaxVersion: TypeAlias = tuple[int, int]
 
 
-def parse_versions(fname: "StrPath") -> Dict[str, Tuple[VersionsTuple, VersionsTuple]]:
+def parse_versions(fname: StrPath) -> dict[str, tuple[MinVersion, MaxVersion]]:
     result = {}
     with open(fname) as f:
         for line in f:
@@ -84,13 +86,13 @@ def parse_versions(fname: "StrPath") -> Dict[str, Tuple[VersionsTuple, VersionsT
 _VERSION_RE = re.compile(r"^([23])\.(\d+)$")
 
 
-def parse_version(v_str: str) -> Tuple[int, int]:
+def parse_version(v_str: str) -> tuple[int, int]:
     m = _VERSION_RE.match(v_str)
     assert m, "invalid version: " + v_str
     return int(m.group(1)), int(m.group(2))
 
 
-def add_files(files: List[str], seen: Set[str], root: str, name: str, args: argparse.Namespace) -> None:
+def add_files(files: list[str], seen: set[str], root: str, name: str, args: argparse.Namespace) -> None:
     """Add all files in package or module represented by 'name' located in 'root'."""
     full = os.path.join(root, name)
     mod, ext = os.path.splitext(name)
@@ -113,7 +115,7 @@ def add_files(files: List[str], seen: Set[str], root: str, name: str, args: argp
 
 class MypyDistConf(NamedTuple):
     module_name: str
-    values: Dict
+    values: dict
 
 
 # The configuration section in the metadata file looks like the following, with multiple module sections possible
@@ -125,7 +127,7 @@ class MypyDistConf(NamedTuple):
 # disallow_untyped_defs = true
 
 
-def add_configuration(configurations: List[MypyDistConf], distribution: str) -> None:
+def add_configuration(configurations: list[MypyDistConf], distribution: str) -> None:
     with open(os.path.join("stubs", distribution, "METADATA.toml")) as f:
         data = dict(tomli.loads(f.read()))
 
@@ -150,10 +152,10 @@ def add_configuration(configurations: List[MypyDistConf], distribution: str) ->
 
 def run_mypy(
     args: argparse.Namespace,
-    configurations: List[MypyDistConf],
+    configurations: list[MypyDistConf],
     major: int,
     minor: int,
-    files: List[str],
+    files: list[str],
     *,
     custom_typeshed: bool = False,
 ) -> int:
@@ -186,7 +188,7 @@ def run_mypy(
 
 def get_mypy_flags(
     args: argparse.Namespace, major: int, minor: int, temp_name: str, *, custom_typeshed: bool = False
-) -> List[str]:
+) -> list[str]:
     flags = [
         "--python-version",
         "%d.%d" % (major, minor),
@@ -213,7 +215,7 @@ def get_mypy_flags(
     return flags
 
 
-def read_dependencies(distribution: str) -> List[str]:
+def read_dependencies(distribution: str) -> list[str]:
     with open(os.path.join("stubs", distribution, "METADATA.toml")) as f:
         data = dict(tomli.loads(f.read()))
     requires = data.get("requires", [])
@@ -229,10 +231,10 @@ def read_dependencies(distribution: str) -> List[str]:
 def add_third_party_files(
     distribution: str,
     major: int,
-    files: List[str],
+    files: list[str],
     args: argparse.Namespace,
-    configurations: List[MypyDistConf],
-    seen_dists: Set[str],
+    configurations: list[MypyDistConf],
+    seen_dists: set[str],
 ) -> None:
     if distribution in seen_dists:
         return
@@ -251,16 +253,16 @@ def add_third_party_files(
         add_configuration(configurations, distribution)
 
 
-def test_third_party_distribution(distribution: str, major: int, minor: int, args: argparse.Namespace) -> Tuple[int, int]:
+def test_third_party_distribution(distribution: str, major: int, minor: int, args: argparse.Namespace) -> tuple[int, int]:
     """Test the stubs of a third-party distribution.
 
     Return a tuple, where the first element indicates mypy's return code
     and the second element is the number of checked files.
     """
 
-    files: List[str] = []
-    configurations: List[MypyDistConf] = []
-    seen_dists: Set[str] = set()
+    files: list[str] = []
+    configurations: list[MypyDistConf] = []
+    seen_dists: set[str] = set()
     add_third_party_files(distribution, major, files, args, configurations, seen_dists)
 
     print(f"testing {distribution} ({len(files)} files)...")
@@ -296,7 +298,7 @@ def main() -> None:
         seen = {"__builtin__", "builtins", "typing"}  # Always ignore these.
 
         # Test standard library files.
-        files: List[str] = []
+        files: list[str] = []
         if major == 2:
             root = os.path.join("stdlib", "@python2")
             for name in os.listdir(root):