title: I can`t authorize with gspread on Python 3
body: <p>I have a simple script with creating googlesheet on Python 3.6 like:</p>
  
  <pre><code>import gspread
  from oauth2client.service_account import ServiceAccountCredentials
  
  mail = 'my_mail@gmail.com'
  secret_file = "C:\\Users\\admin\\Downloads\\my_file.json"
  
  SCOPES = ['https://spreadsheets.google.com/feeds']
  
  credentials = ServiceAccountCredentials.from_json_keyfile_name(secret_file, SCOPES)
  
  gss_client = gspread.authorize(credentials)
  gss = gss_client.open('test-doc')
  worksheet = gss.sheet1
  </code></pre>
  
  <p>But when I run it, I have a type-error:</p>
  
  <blockquote>
    <blockquote>
      <p>Traceback (most recent call last):   File
      "C:/Users/admin/PycharmProjects/untitled/fjhdshfjs.py", line 11, in
       ...
      "C:\Users\admin\AppData\Local\Programs\Python\Python36\lib\base64.py",
      line 58, in b64encode
          encoded = binascii.b2a_base64(s, newline=False) TypeError: a bytes-like object is required, not 'str'</p>
    </blockquote>
  </blockquote>
  
  <p>Early, this trouble could be resolved with encoding:</p>
  
  <pre><code>credentials = SignedJwtAssertionCredentials(json_key['client_email'], json_key['private_key'].encode(), scope)
  </code></pre>
  
  <p>But now, it doesn`t work after <em>moving SignedJwtAssertionCredentials onto oauth2client.service_account.ServiceAccountCredentials</em> </p>
  
  <p>Maybe somebody can help me? </p>