Remove the setuptools and CFFI requirements, which break pip install -U

PyPy ships with CFFI, but pip install -U will upgrade it if a newer CFFI
is available, which will break the CFFI installation (and PyPy itself).

setup.py

@@ -35,7 +35,6 @@
 VECTORS_DEPENDENCY = "cryptography_vectors=={0}".format(about['__version__'])
 
 requirements = [
-    CFFI_DEPENDENCY,
     "pyasn1",
     SIX_DEPENDENCY,
     SETUPTOOLS_DEPENDENCY
@@ -44,6 +43,9 @@
 if sys.version_info < (3, 4):
     requirements.append("enum34")
 
+if platform.python_implementation() != "PyPy":
+    requirements.append(CFFI_DEPENDENCY)
+
 # If you add a new dep here you probably need to add it in the tox.ini as well
 test_requirements = [
     "pytest",