Add check-manifest tool and MANIFEST.in

MANIFEST.in

@@ -0,0 +1,5 @@
+include CODE_OF_CONDUCT.md
+include HISTORY.md
+include MANIFEST.in
+include Makefile
+include sample.json

pyproject.toml

@@ -31,6 +31,7 @@ fx-crash-sig = "fx_crash_sig.cmd_get_crash_sig:cmdline"
 [project.optional-dependencies]
 dev = [
     "build",
+    "check-manifest",
     "pytest",
     "ruff",
     "tox",