Add --additional-flags argument (#50)

So we can use it in typeshed to disable bytearray/memoryview promotions

README.md

@@ -32,13 +32,14 @@ The hardcoded list is in theory susceptible to bitrot, but if you pass e.g. the
 usage: mypy_primer [-h] [--new NEW] [--old OLD] [--repo REPO]
                    [--mypyc-compile-level MYPYC_COMPILE_LEVEL]
                    [--custom-typeshed-repo CUSTOM_TYPESHED_REPO] [--new-typeshed NEW_TYPESHED]
-                   [--old-typeshed OLD_TYPESHED] [-k PROJECT_SELECTOR] [-p LOCAL_PROJECT]
-                   [--expected-success] [--project-date PROJECT_DATE] [--num-shards NUM_SHARDS]
+                   [--old-typeshed OLD_TYPESHED] [--additional-flags [ADDITIONAL_FLAGS ...]]
+                   [-k PROJECT_SELECTOR] [-p LOCAL_PROJECT] [--expected-success]
+                   [--project-date PROJECT_DATE] [--num-shards NUM_SHARDS]
                    [--shard-index SHARD_INDEX] [-o {full,diff,concise}] [--old-success]
                    [--coverage] [--bisect] [--bisect-output BISECT_OUTPUT] [-j CONCURRENCY]
                    [--debug] [--base-dir BASE_DIR] [--clear]
 
-optional arguments:
+options:
   -h, --help            show this help message and exit
 
 mypy:
@@ -58,6 +59,8 @@ mypy:
   --old-typeshed OLD_TYPESHED
                         old typeshed version, defaults to mypy's (anything commit-ish, or
                         isoformatted date)
+  --additional-flags [ADDITIONAL_FLAGS ...]
+                        additional flags to pass to mypy
 
 project selection:
   -k PROJECT_SELECTOR, --project-selector PROJECT_SELECTOR

mypy_primer.py

@@ -366,7 +366,7 @@ def get_mypy_cmd(self, mypy: str | Path, additional_flags: Sequence[str] = ()) -
         return mypy_cmd
 
     async def run_mypy(self, mypy: str | Path, typeshed_dir: Path | None) -> MypyResult:
-        additional_flags = []
+        additional_flags = list(ARGS.additional_flags)
         env = os.environ.copy()
         env["MYPY_FORCE_COLOR"] = "1"
 
@@ -809,6 +809,12 @@ def parse_options(argv: list[str]) -> argparse.Namespace:
         "--old-typeshed",
         help="old typeshed version, defaults to mypy's (anything commit-ish, or isoformatted date)",
     )
+    mypy_group.add_argument(
+        "--additional-flags",
+        help="additional flags to pass to mypy",
+        nargs="*",
+        default=[],
+    )
 
     proj_group = parser.add_argument_group("project selection")
     proj_group.add_argument(