Don't rely on SERVER_SOFTWARE being set for AppEngine (#1704)

src/urllib3/contrib/_appengine_environ.py

@@ -6,27 +6,31 @@
 
 
 def is_appengine():
-    return is_local_appengine() or is_prod_appengine() or is_prod_appengine_mvms()
+    return "APPENGINE_RUNTIME" in os.environ
 
 
 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 is_appengine() 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 is_appengine() 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

src/urllib3/contrib/appengine.py

@@ -108,13 +108,6 @@ def __init__(
                 "URLFetch is not available in this environment."
             )
 
-        if is_prod_appengine_mvms():
-            raise AppEnginePlatformError(
-                "Use normal urllib3.PoolManager instead of AppEngineManager"
-                "on Managed VMs, as using URLFetch is not necessary in "
-                "this environment."
-            )
-
         warnings.warn(
             "urllib3 is using URLFetch on Google App Engine sandbox instead "
             "of sockets. To use sockets directly instead of URLFetch see "