bpo-27535: Fix memory leak with warnings ignore (#4489)

The warnings module doesn't leak memory anymore in the hidden
warnings registry for the "ignore" action of warnings filters.

The warn_explicit() function doesn't add the warning key to the
registry anymore for the "ignore" action.

Lib/warnings.py

@@ -364,7 +364,6 @@ def warn_explicit(message, category, filename, lineno,
         action = defaultaction
     # Early exit actions
     if action == "ignore":
-        registry[key] = 1
         return
 
     # Prime the linecache for formatting, in case the