gh-109413: libregrtest: enable mypy's --strict-optional check on most files

Lib/test/libregrtest/mypy.ini

@@ -25,7 +25,7 @@ warn_return_any = False
 disable_error_code = return
 
 # Enable --strict-optional for these ASAP:
-[mypy-Lib.test.libregrtest.main.*,Lib.test.libregrtest.run_workers.*,Lib.test.libregrtest.worker.*,Lib.test.libregrtest.single.*,Lib.test.libregrtest.results.*,Lib.test.libregrtest.utils.*]
+[mypy-Lib.test.libregrtest.main.*,Lib.test.libregrtest.run_workers.*]
 strict_optional = False
 
 # Various internal modules that typeshed deliberately doesn't have stubs for:

Lib/test/libregrtest/results.py

@@ -31,7 +31,7 @@ def __init__(self):
 
         self.interrupted: bool = False
         self.worker_bug: bool = False
-        self.test_times: list[tuple[float, TestName]] = []
+        self.test_times: list[tuple[float | None, TestName]] = []
         self.stats = TestStats()
         # used by --junit-xml
         self.testsuite_xml: list = []

Lib/test/libregrtest/runtests.py

@@ -93,7 +93,7 @@ class RunTests:
     python_cmd: tuple[str, ...] | None
     randomize: bool
     random_seed: int | str
-    json_file: JsonFile | None
+    json_file: JsonFile
 
     def copy(self, **override):
         state = dataclasses.asdict(self)

Lib/test/libregrtest/single.py

@@ -237,11 +237,11 @@ def _runtest(result: TestResult, runtests: RunTests) -> None:
     output_on_failure = runtests.output_on_failure
     timeout = runtests.timeout
 
-    use_timeout = (
-        timeout is not None and threading_helper.can_start_thread
-    )
-    if use_timeout:
+    if timeout is not None and threading_helper.can_start_thread:
+        use_timeout = True
         faulthandler.dump_traceback_later(timeout, exit=True)
+    else:
+        use_timeout = False
 
     try:
         setup_tests(runtests)

Lib/test/libregrtest/utils.py

@@ -377,10 +377,19 @@ def get_temp_dir(tmp_dir: StrPath | None = None) -> StrPath:
                         # Python out of the source tree, especially when the
                         # source tree is read only.
                         tmp_dir = sysconfig.get_config_var('srcdir')
+                        if not tmp_dir:
+                            raise RuntimeError(
+                                "Could not determine the correct value for tmp_dir"
+                            )
                 tmp_dir = os.path.join(tmp_dir, 'build')
             else:
                 # WASI platform
                 tmp_dir = sysconfig.get_config_var('projectbase')
+                if tmp_dir is None:
+                    raise RuntimeError(
+                        "sysconfig.get_config_var('projectbase') "
+                        "unexpectedly returned `None` on WASI"
+                    )
                 tmp_dir = os.path.join(tmp_dir, 'build')
 
                 # When get_temp_dir() is called in a worker process,