gh-109413: Improve mypy config for libregrtest (#109518)

Improve the mypy config file for libregrtest

Lib/test/libregrtest/mypy.ini

@@ -1,12 +1,10 @@
 # Config file for running mypy on libregrtest.
-#
-# Note: mypy can't be run on libregrtest from the CPython repo root.
-# If you try to do so, mypy will complain
-# about the entire `Lib/` directory "shadowing the stdlib".
-# Instead, `cd` into `Lib/test`, then run `mypy --config-file libregrtest/mypy.ini`.
+# Run mypy by invoking `mypy --config-file Lib/test/libregrtest/mypy.ini`
+# on the command-line from the repo root
 
 [mypy]
-packages = libregrtest
+files = Lib/test/libregrtest
+explicit_package_bases = True
 python_version = 3.11
 platform = linux
 pretty = True
@@ -17,7 +15,6 @@ strict = True
 
 # Various stricter settings that we can't yet enable
 # Try to enable these in the following order:
-strict_optional = False
 disallow_any_generics = False
 disallow_incomplete_defs = False
 disallow_untyped_calls = False
@@ -27,21 +24,10 @@ 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.*]
+strict_optional = False
+
 # Various internal modules that typeshed deliberately doesn't have stubs for:
-[mypy-_abc.*]
-ignore_missing_imports = True
-
-[mypy-_opcode.*]
-ignore_missing_imports = True
-
-[mypy-_overlapped.*]
-ignore_missing_imports = True
-
-[mypy-_testcapi.*]
-ignore_missing_imports = True
-
-[mypy-_testinternalcapi.*]
-ignore_missing_imports = True
-
-[mypy-test.*]
+[mypy-_abc.*,_opcode.*,_overlapped.*,_testcapi.*,_testinternalcapi.*,test.*]
 ignore_missing_imports = True