Gss api delegate (#22)

Updated changelog.
Added function for setting GSS API delegation option on session. Added GSSAPI authentication tests.
Updated import privkey function signatures.
Added agent authentication test. Updated import privkey functions to default to empty passphrase.

Author: pkittenis

Changelog.rst

@@ -2,6 +2,16 @@ Change Log
 =============
 
 
+0.6.0
++++++
+
+Changes
+--------
+
+* Added function for setting GSS-API credentials delegation option to session.
+* Updated error handling for all user authentication session functions to raise specific authentication errors.
+
+
 0.5.0
 +++++
 
@@ -36,6 +46,7 @@ Packaging
 * Added OSX 10.14 and 10.15 binary wheels for Python 3.8
 * Added Windows 64-bit binary wheels for Python 3.6/3.7/3.8
 
+
 0.3.0
 ++++++++
 

ssh/key.c

@@ -134,7 +134,7 @@ def generate(KeyType key_type, int bits):
     return key
 
 
-def import_privkey_base64(bytes b64_key, passphrase=None):
+def import_privkey_base64(bytes b64_key, passphrase=b''):
     cdef const_char *c_key = b64_key
     cdef bytes b_passphrase
     cdef const_char *c_passphrase = NULL
@@ -153,7 +153,7 @@ def import_privkey_base64(bytes b64_key, passphrase=None):
     return key
 
 
-def import_privkey_file(filepath, passphrase=None):
+def import_privkey_file(filepath, passphrase=b''):
     cdef bytes b_passphrase
     cdef bytes b_filepath = to_bytes(filepath)
     cdef const_char *c_passphrase = NULL