Fail on PyPy in main instead of setup.py (#20389)

Follow-up for #20384

Author: ilevkivskyi

mypy/main.py

@@ -4,6 +4,7 @@
 
 import argparse
 import os
+import platform
 import subprocess
 import sys
 import time
@@ -39,6 +40,13 @@
 if TYPE_CHECKING:
     from _typeshed import SupportsWrite
 
+if platform.python_implementation() == "PyPy":
+    sys.stderr.write(
+        "ERROR: Running mypy on PyPy is not supported yet.\n"
+        "To type-check a PyPy library please use an equivalent CPython version,\n"
+        "see https://github.com/mypyc/librt/issues/16 for possible workarounds.\n"
+    )
+    sys.exit(2)
 
 orig_stat: Final = os.stat
 MEM_PROFILE: Final = False  # If True, dump memory profile

setup.py

@@ -5,22 +5,13 @@
 import glob
 import os
 import os.path
-import platform
 import sys
 from typing import TYPE_CHECKING, Any
 
 if sys.version_info < (3, 9, 0):  # noqa: UP036, RUF100
     sys.stderr.write("ERROR: You need Python 3.9 or later to use mypy.\n")
     exit(1)
 
-if platform.python_implementation() == "PyPy":
-    sys.stderr.write(
-        "ERROR: Running mypy on PyPy is not supported yet.\n"
-        "To type-check a PyPy library please use an equivalent CPython version,\n"
-        "see https://github.com/mypyc/librt/issues/16 for possible workarounds.\n"
-    )
-    exit(1)
-
 # we'll import stuff from the source tree, let's ensure is on the sys path
 sys.path.insert(0, os.path.dirname(os.path.realpath(__file__)))