Remove version number from an error message so that it makes sense (#4188)

diff --git a/setup.py b/setup.py
index b655816..4d7f5c2 100644
--- a/setup.py
+++ b/setup.py
@@ -48,8 +48,8 @@
 if platform.python_implementation() == "PyPy":
     if sys.pypy_version_info < (5, 3):
         raise RuntimeError(
-            "cryptography 1.9 is not compatible with PyPy < 5.3. Please "
-            "upgrade PyPy to use this library."
+            "cryptography is not compatible with PyPy < 5.3. Please upgrade "
+            "PyPy to use this library."
         )
 else:
     setup_requirements.append("cffi>=1.7,!=1.11.3")