Fixed bugs in gwpy.io.kerberos for python3

This PR fixes bugs in kerberos.kinit on python3, just a simple type issue with str vs bytes. I took this opportunity to make some small, backwards-compatible internal improvements to that method.

gwpy/io/tests/test_kerberos.py

@@ -34,7 +34,7 @@ __author__ = 'Duncan Macleod <duncan.macleod@ligo.org>'
 _KTNAME = os.environ.get('KRB5_KTNAME', '_NO KT_')
 
 # mocked klist output
-KLIST = """Keytab name: FILE:/test.keytab
+KLIST = b"""Keytab name: FILE:/test.keytab
 KVNO Principal
 ---- -------------------------------
    1 albert.einstein@LIGO.ORG
@@ -82,8 +82,10 @@ def test_parse_keytab(mocked_popen):
 @mock.patch('getpass.getpass', return_value='test')
 @mock.patch('gwpy.io.kerberos.input', return_value='rainer.weiss')
 def test_kinit(raw_input_, getpass, mocked_popen, which, capsys):
+    mocked_popen.return_value.poll.return_value = 0
+
     # default popen kwargs
-    popen_kwargs = {'stdin': -1, 'stderr': -1, 'stdout': -1, 'env': None}
+    popen_kwargs = {'stdin': -1, 'stdout': -1, 'env': None}
 
     # pass username and password, and kinit exe path
     io_kerberos.kinit(username='albert.einstein', password='test',
@@ -95,7 +97,7 @@ def test_kinit(raw_input_, getpass, mocked_popen, which, capsys):
         'Kerberos ticket generated for albert.einstein@LIGO.ORG\n')
 
     # configure klisting (remove Drever)
-    mock_popen_return(mocked_popen, out=KLIST.rsplit('\n', 1)[0])
+    mock_popen_return(mocked_popen, out=KLIST.rsplit(b'\n', 1)[0])
     os.environ['KRB5_KTNAME'] = '/test.keytab'
 
     # test keytab from environment not found (default) prompts user