mypy_test: Add two more config options (#7560)

`--enable-error-code ignore-without-code` means that mypy will ignore any type: ignore comments that don't have mypy error codes. It doesn't appear to have any effect on type: ignore comments that mypy_test doesn't use (e.g. because they're pyright- or stubtest-specific).

`--strict-equality` means that mypy will raise errors if we do something silly like `if sys.version_info == "linux"`. flake8-pyi should also check this for us, but I don't see any reason not to have mypy check this as well.

tests/mypy_test.py

@@ -207,6 +207,9 @@ def get_mypy_flags(args, major: int, minor: int, temp_name: str, *, custom_types
         "--warn-incomplete-stub",
         "--show-error-codes",
         "--no-error-summary",
+        "--enable-error-code",
+        "ignore-without-code",
+        "--strict-equality",
     ]
     if custom_typeshed:
         # Setting custom typeshed dir prevents mypy from falling back to its bundled