Update NumPy documentation (1.21.4)

Fixes #1650.

lib/docs/filters/sphinx/clean_html.rb

@@ -59,6 +59,7 @@ module Docs
 
         css('dt').each do |node|
           next if current_url.host == 'matplotlib.org'
+          next if current_url.host == 'numpy.org'
           next unless node['id'] || node.at_css('code, .classifier')
           links = []
           links << node.children.last.remove while node.children.last.try(:name) == 'a'

lib/docs/scrapers/numpy.rb

@@ -27,7 +27,7 @@ module Docs
     HTML
 
     version '1.21' do
-      self.release = '1.21.1'
+      self.release = '1.21.4'
       self.base_url = "https://numpy.org/doc/#{self.version}/"
       options[:container] = nil
     end