add key types consts (#776)

diff --git a/src/OpenSSL/crypto.py b/src/OpenSSL/crypto.py
index 255ec8f..12e92df 100644
--- a/src/OpenSSL/crypto.py
+++ b/src/OpenSSL/crypto.py
@@ -71,6 +71,8 @@
 
 TYPE_RSA = _lib.EVP_PKEY_RSA
 TYPE_DSA = _lib.EVP_PKEY_DSA
+TYPE_DH = _lib.EVP_PKEY_DH
+TYPE_EC = _lib.EVP_PKEY_EC
 
 
 class Error(Exception):