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/Requester.py

@@ -80,10 +80,7 @@ class RequestsResponse:
         self.text = r.text
 
     def getheaders(self):
-        if atLeastPython3:
-            return list(self.headers.items())
-        else:
-            return six.iteritems(self.headers)
+        return six.iteritems(self.headers)
 
     def read(self):
         return self.text
@@ -231,10 +228,7 @@ class Requester:
 
         if password is not None:
             login = login_or_token
-            if atLeastPython3:
-                self.__authorizationHeader = "Basic " + base64.b64encode((login + ":" + password).encode("utf-8")).decode("utf-8").replace('\n', '')  # pragma no cover (Covered by Authentication.testAuthorizationHeaderWithXxx with Python 3)
-            else:
-                self.__authorizationHeader = "Basic " + base64.b64encode(login + ":" + password).replace('\n', '')
+            self.__authorizationHeader = "Basic " + base64.b64encode((login + ":" + password).encode("utf-8")).decode("utf-8").replace('\n', '')
         elif login_or_token is not None:
             token = login_or_token
             self.__authorizationHeader = "token " + token
@@ -323,8 +317,8 @@ class Requester:
         if len(data) == 0:
             return None
         else:
-            if atLeastPython3 and isinstance(data, bytes):  # pragma no branch (Covered by Issue142.testDecodeJson with Python 3)
-                data = data.decode("utf-8")  # pragma no cover (Covered by Issue142.testDecodeJson with Python 3)
+            if isinstance(data, bytes):
+                data = data.decode("utf-8")
             try:
                 return json.loads(data)
             except ValueError: