Update vendored requests to 2.25.1

Updates certifi to 2021.5.30
Updates chardet to 4.0.0
Updates idna to 2.10
Updates urllib3 to 1.26.13

libs/common/urllib3/contrib/_appengine_environ.py

@@ -6,25 +6,31 @@ import os
 
 
 def is_appengine():
-    return (is_local_appengine() or
-            is_prod_appengine() or
-            is_prod_appengine_mvms())
+    return is_local_appengine() or is_prod_appengine()
 
 
 def is_appengine_sandbox():
-    return is_appengine() and not is_prod_appengine_mvms()
+    """Reports if the app is running in the first generation sandbox.
+
+    The second generation runtimes are technically still in a sandbox, but it
+    is much less restrictive, so generally you shouldn't need to check for it.
+    see https://cloud.google.com/appengine/docs/standard/runtimes
+    """
+    return is_appengine() and os.environ["APPENGINE_RUNTIME"] == "python27"
 
 
 def is_local_appengine():
-    return ('APPENGINE_RUNTIME' in os.environ and
-            'Development/' in os.environ['SERVER_SOFTWARE'])
+    return "APPENGINE_RUNTIME" in os.environ and os.environ.get(
+        "SERVER_SOFTWARE", ""
+    ).startswith("Development/")
 
 
 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())
+    return "APPENGINE_RUNTIME" in os.environ and os.environ.get(
+        "SERVER_SOFTWARE", ""
+    ).startswith("Google App Engine/")
 
 
 def is_prod_appengine_mvms():
-    return os.environ.get('GAE_VM', False) == 'true'
+    """Deprecated."""
+    return False