PythonDependency: massive refactoring.

Closes Homebrew#24842.

Library/Homebrew/dependency_collector.rb

@@ -74,9 +74,6 @@ def parse_string_spec(spec, tags)
     if tags.empty?
       Dependency.new(spec, tags)
     elsif (tag = tags.first) && LANGUAGE_MODULES.include?(tag)
-      # Next line only for legacy support of `depends_on 'module' => :python`
-      # It should be replaced by `depends_on :python => 'module'`
-      return PythonDependency.new("2", Array(spec)) if tag == :python
       LanguageModuleDependency.new(tag, spec)
     else
       Dependency.new(spec, tags)
@@ -105,8 +102,9 @@ def parse_symbol_spec(spec, tags)
     when :clt        then CLTDependency.new(tags)
     when :arch       then ArchRequirement.new(tags)
     when :hg         then MercurialDependency.new(tags)
-    when :python, :python2 then PythonDependency.new("2", tags)
-    when :python3    then PythonDependency.new("3", tags)
+    # python2 is deprecated
+    when :python, :python2 then PythonDependency.new(tags)
+    when :python3    then Python3Dependency.new(tags)
     # Tiger's ld is too old to properly link some software
     when :ld64       then LD64Dependency.new if MacOS.version < :leopard
     when :ant        then ant_dep(spec, tags)

Library/Homebrew/formula.rb

@@ -340,24 +340,14 @@ def std_cmake_args
     ]
   end
 
-  # Install python bindings inside of a block given to this method and/or
-  # call python so: `system python, "setup.py", "install", "--prefix=#{prefix}"
-  # Note that there are no quotation marks around python!
-  # <https://github.com/Homebrew/homebrew/wiki/Homebrew-and-Python>
-  def python(options={:allowed_major_versions => [2, 3]}, &block)
-    require 'python_helper'
-    python_helper(options, &block)
-  end
-
-  # Explicitly only execute the block for 2.x (if a python 2.x is available)
-  def python2 &block
-    python(:allowed_major_versions => [2], &block)
-  end
-
-  # Explicitly only execute the block for 3.x (if a python 3.x is available)
-  def python3 &block
-    python(:allowed_major_versions => [3], &block)
-  end
+  # Deprecated
+  def python(options={}, &block)
+    opoo 'Formula#python is deprecated and will go away shortly.'
+    block.call if block_given?
+    PythonDependency.new
+  end
+  alias_method :python2, :python
+  alias_method :python3, :python
 
   # Generates a formula's ruby class name from a formula's name
   def self.class_s name