Enforce that PEM strings (certs, keys, CSR) are internally passed as str, not bytes

This was already true in most places but not 100%, leading to lots of redundant checks and conversions.

lemur/plugins/lemur_openssl/plugin.py

@@ -44,14 +44,9 @@ def create_pkcs12(cert, chain, p12_tmp, key, alias, passphrase):
     :param alias:
     :param passphrase:
     """
-    if isinstance(cert, bytes):
-        cert = cert.decode('utf-8')
-
-    if isinstance(chain, bytes):
-        chain = chain.decode('utf-8')
-
-    if isinstance(key, bytes):
-        key = key.decode('utf-8')
+    assert isinstance(cert, str)
+    assert isinstance(chain, str)
+    assert isinstance(key, str)
 
     with mktempfile() as key_tmp:
         with open(key_tmp, 'w') as f: