virtualenv: support mojave and high sierra system Python

On High Sierra and Mojave, virtualenv 16.x does not build when used with
system Python
Fall back to an older virtualenv version, which is known to work.

Library/Homebrew/language/python.rb

@@ -93,8 +93,13 @@ module Language
       def self.included(base)
         base.class_eval do
           resource "homebrew-virtualenv" do
-            url PYTHON_VIRTUALENV_URL
-            sha256 PYTHON_VIRTUALENV_SHA256
+            if MacOS.version > :mojave
+              url PYTHON_VIRTUALENV_URL
+              sha256 PYTHON_VIRTUALENV_SHA256
+            else
+              url PYTHON_VIRTUALENV_URL_MOJAVE
+              sha256 PYTHON_VIRTUALENV_SHA256_MOJAVE
+            end
           end
         end
       end

Library/Homebrew/language/python_virtualenv_constants.rb

@@ -6,3 +6,10 @@ PYTHON_VIRTUALENV_URL =
   "/virtualenv-16.7.4.tar.gz"
 PYTHON_VIRTUALENV_SHA256 =
   "94a6898293d07f84a98add34c4df900f8ec64a570292279f6d91c781d37fd305"
+
+PYTHON_VIRTUALENV_URL_MOJAVE =
+  "https://files.pythonhosted.org/packages/b1/72" \
+  "/2d70c5a1de409ceb3a27ff2ec007ecdd5cc52239e7c74990e32af57affe9" \
+  "/virtualenv-15.2.0.tar.gz"
+PYTHON_VIRTUALENV_SHA256_MOJAVE =
+  "1d7e241b431e7afce47e77f8843a276f652699d1fa4f93b9d8ce0076fd7b0b54"