Drop httplib for requests to finish work of #664 (#998)

#664 got rid of most of httplib and replaced it with requests.
This is the last place it was used. It makes a more coherent code base I think.

Tests are ok locally.

github/MainClass.py

@@ -53,7 +53,7 @@ import urllib
 import pickle
 import time
 import sys
-from httplib import HTTPSConnection
+import requests
 import jwt
 
 from Requester import Requester, json
@@ -775,25 +775,19 @@ class GithubIntegration(object):
         body = None
         if user_id:
             body = json.dumps({"user_id": user_id})
-        conn = HTTPSConnection("api.github.com")
-        conn.request(
-            method="POST",
-            url="/installations/{}/access_tokens".format(installation_id),
-            headers={
-                "Authorization": "Bearer {}".format(self.create_jwt()),
-                "Accept": Consts.mediaTypeIntegrationPreview,
-                "User-Agent": "PyGithub/Python"
-            },
-            body=body
-        )
-        response = conn.getresponse()
-        response_text = response.read()
+        response = requests.post("https://api.github.com/installations/{}/access_tokens".format(installation_id),
+                                 headers={
+                                     "Authorization": "Bearer {}".format(self.create_jwt()),
+                                     "Accept": Consts.mediaTypeIntegrationPreview,
+                                     "User-Agent": "PyGithub/Python"
+                                 },
+                                 body=body)
+        response_text = response.text
 
         if atLeastPython3:
             response_text = response_text.decode('utf-8')
 
-        conn.close()
-        if response.status == 201:
+        if response.status_code == 201:
             data = json.loads(response_text)
             return InstallationAuthorization.InstallationAuthorization(
                 requester=None,  # not required, this is a NonCompletableGithubObject
@@ -801,17 +795,17 @@ class GithubIntegration(object):
                 attributes=data,
                 completed=True
             )
-        elif response.status == 403:
+        elif response.status_code == 403:
             raise GithubException.BadCredentialsException(
-                status=response.status,
+                status=response.status_code,
                 data=response_text
             )
-        elif response.status == 404:
+        elif response.status_code == 404:
             raise GithubException.UnknownObjectException(
-                status=response.status,
+                status=response.status_code,
                 data=response_text
             )
         raise GithubException.GithubException(
-            status=response.status,
+            status=response.status_code,
             data=response_text
         )