add (overwrite with) mypy stubs, if available

stdlib/3/distutils/errors.pyi

@@ -0,0 +1,4 @@
+import typing
+
+class DistutilsError(Exception): ...
+class DistutilsExecError(DistutilsError): ...

stdlib/3/distutils/spawn.pyi

@@ -0,0 +1,6 @@
+from typing import List
+
+# In Python, arguments have integer default values
+def spawn(cmd: List[str], search_path: bool = True, verbose: bool = False,
+           dry_run: bool = False) -> None: ...
+def find_executable(executable: str, path: str = None) -> str: ...