Print error conditionally

Author: ilevkivskyi

mypy/build.py

@@ -43,12 +43,15 @@
 try:
     from librt.internal import cache_version
 except ImportError:
-    print(
-        "Running mypy on PyPy is not supported yet."
-        "To type-check a PyPy library please run mypy on an equivalent CPython version,"
-        "see https://github.com/mypyc/librt/issues/16 for possible workarounds."
-    )
-    sys.exit(2)
+    if platform.python_implementation() == "PyPy":
+        print(
+            "Running mypy on PyPy is not supported yet."
+            "To type-check a PyPy library please run mypy on an equivalent CPython version,"
+            "see https://github.com/mypyc/librt/issues/16 for possible workarounds."
+        )
+        sys.exit(2)
+    else:
+        raise
 
 import mypy.semanal_main
 from mypy.cache import CACHE_VERSION, CacheMeta, ReadBuffer, WriteBuffer, write_json