Update requests to 2.21.0

Also updates:
- certifi-2018.11.29
- chardet-3.0.4
- idna-2.8
- urllib3-1.24.1

libs/urllib3/contrib/_appengine_environ.py

@@ -0,0 +1,30 @@
+"""
+This module provides means to detect the App Engine environment.
+"""
+
+import os
+
+
+def is_appengine():
+    return (is_local_appengine() or
+            is_prod_appengine() or
+            is_prod_appengine_mvms())
+
+
+def is_appengine_sandbox():
+    return is_appengine() and not is_prod_appengine_mvms()
+
+
+def is_local_appengine():
+    return ('APPENGINE_RUNTIME' in os.environ and
+            'Development/' in os.environ['SERVER_SOFTWARE'])
+
+
+def is_prod_appengine():
+    return ('APPENGINE_RUNTIME' in os.environ and
+            'Google App Engine/' in os.environ['SERVER_SOFTWARE'] and
+            not is_prod_appengine_mvms())
+
+
+def is_prod_appengine_mvms():
+    return os.environ.get('GAE_VM', False) == 'true'