Improve Pitchfork::Info IO tracking

First avoid setting the IO instance as weakmap value as it
requires more work for Ruby and also can run into
ruby/ruby#7540

Second allow both `$std*` and `STD*` to remain open.

lib/pitchfork/info.rb

@@ -13,7 +13,7 @@ class << self
 
       def keep_io(io)
         raise ArgumentError, "#{io.inspect} doesn't respond to :to_io" unless io.respond_to?(:to_io)
-        @kept_ios[io] = io
+        @kept_ios[io] = true
         io
       end
 
@@ -22,9 +22,9 @@ def keep_ios(ios)
       end
 
       def close_all_ios!
-        ignored_ios = [$stdin, $stdout, $stderr]
+        ignored_ios = [$stdin, $stdout, $stderr, STDIN, STDOUT, STDERR].uniq.compact
 
-        @kept_ios.each_value do |io_like|
+        @kept_ios.each_key do |io_like|
           ignored_ios << (io_like.is_a?(IO) ? io_like : io_like.to_io)
         end