Remove some uses of atLeastPython3 (#1191)

There are a number of call sites that would behave differently depending
on Python 2 or 3. A fair amount of them are left over from Python 3.2 or
Python 2.6 were the current versions, and it was much harder to write
compatible code for both versions. Happily, that is now in the past, so
refactor some of them out.

github/ContentFile.py

@@ -39,9 +39,6 @@ import github.GithubObject
 import github.Repository
 
 
-atLeastPython3 = sys.hexversion >= 0x03000000
-
-
 class ContentFile(github.GithubObject.CompletableGithubObject):
     """
     This class represents ContentFiles. The reference can be found here https://developer.github.com/v3/repos/contents/#get-contents
@@ -61,11 +58,7 @@ class ContentFile(github.GithubObject.CompletableGithubObject):
     @property
     def decoded_content(self):
         assert self.encoding == "base64", "unsupported encoding: %s" % self.encoding
-        if atLeastPython3:
-            content = bytearray(self.content, "utf-8")  # pragma no cover (covered by tests with Python 3.2)
-        else:
-            content = self.content
-        return base64.b64decode(content)
+        return base64.b64decode(bytearray(self.content, "utf-8"))
 
     @property
     def download_url(self):