Detect empty HTTPS contexts.

Some users are relying solely on the certifi package to provide their CA certs, as requests does this by default.

This patch detects this situation and emits a clear warning as well as importing certifi to work around the situation..

Fixes josegonzalez#162 .

github_backup/github_backup.py

@@ -15,6 +15,7 @@
 import re
 import select
 import socket
+import ssl
 import subprocess
 import sys
 import time
@@ -36,6 +37,18 @@
 FILE_URI_PREFIX = "file://"
 logger = logging.getLogger(__name__)
 
+https_ctx = ssl.create_default_context()
+if not https_ctx.get_ca_certs():
+    import warnings
+    warnings.warn('\n\nYOUR DEFAULT CA CERTS ARE EMPTY.\n' +
+                  'PLEASE POPULATE ANY OF:' +
+                  ''.join([
+                    '\n - ' + x
+                    for x in ssl.get_default_verify_paths()
+                    if type(x) is str
+                  ]) + '\n', stacklevel=2)
+    import certifi
+    https_ctx = ssl.create_default_context(cafile=certifi.where())
 
 def logging_subprocess(
     popenargs, stdout_log_level=logging.DEBUG, stderr_log_level=logging.ERROR, **kwargs
@@ -666,7 +679,7 @@ def _get_response(request, auth, template):
     while True:
         should_continue = False
         try:
-            r = urlopen(request)
+            r = urlopen(request, context=https_ctx)
         except HTTPError as exc:
             errors, should_continue = _request_http_error(exc, auth, errors)  # noqa
             r = exc