Add setup.py to selfcheck.

Enhance the setup.py types.

mypy's setup.py is used as inspiration for other setuptools using Python
projects that want to produce mypyc compiled binary wheels. Therefore it
should also be typechecked and held to a higher standard.

runtests.py

@@ -56,14 +56,10 @@
     # Self type check
     "self": [
         executable,
-        "-m",
-        "mypy",
-        "--config-file",
-        "mypy_self_check.ini",
-        "-p",
-        "mypy",
-        "-p",
-        "mypyc",
+        "-c",
+        "from mypy.main import main as mypy; "
+        "mypy(args=['--config-file', 'mypy_self_check.ini', '-p', 'mypy', '-p', 'mypyc']); "
+        "mypy(args=['--config-file', 'mypy_self_check.ini', 'setup.py'])"
     ],
     # Lint
     "lint": ["pre-commit", "run", "--all-files"],

setup.py

@@ -31,7 +31,7 @@ def is_list_of_setuptools_extension(items: list[Any]) -> TypeGuard[list[Extensio
     return all(isinstance(item, Extension) for item in items)
 
 
-def find_package_data(base, globs, root="mypy"):
+def find_package_data(base: str, globs: list[str], root: str = "mypy") -> list[str]:
     """Find all interesting data files, for setup(package_data=)
 
     Arguments:
@@ -52,13 +52,13 @@ def find_package_data(base, globs, root="mypy"):
 
 
 class CustomPythonBuild(build_py):
-    def pin_version(self):
+    def pin_version(self) -> None:
         path = os.path.join(self.build_lib, "mypy")
         self.mkpath(path)
         with open(os.path.join(path, "version.py"), "w") as stream:
             stream.write(f'__version__ = "{version}"\n')
 
-    def run(self):
+    def run(self) -> None:
         self.execute(self.pin_version, ())
         build_py.run(self)
 
@@ -153,10 +153,10 @@ def run(self):
         # our Appveyor builds run out of memory sometimes.
         multi_file=sys.platform == "win32" or force_multifile,
     )
-    assert is_list_of_setuptools_extension(ext_modules), "Expected mypycify to use setuptools"
 
 else:
     ext_modules = []
 
+assert is_list_of_setuptools_extension(ext_modules), "Expected mypycify to use setuptools"
 
 setup(version=version, ext_modules=ext_modules, cmdclass=cmdclass)