appscale-tools: change PyPi URL to pythonhosted.org

- As these are in an `unless OS.mac?` condition, they don't apply to the
  upstream Homebrew formula. These changes fix the following `brew
  audit` errors:
  ```
    * C: 319: col 7: https://pypi.python.org/packages/a5/a5/0830cfe34a4cfd0d1c3c8b614ede1edb2aaf999091ac8548dd19cb352e79/SecretStorage-2.3.1.tar.gz should be `https://files.pythonhosted.org/packages/a5/a5/0830cfe34a4cfd0d1c3c8b614ede1edb2aaf999091ac8548dd19cb352e79/SecretStorage-2.3.1.tar.gz`
  * C: 324: col 7: https://pypi.python.org/packages/3c/ec/a94f8cf7274ea60b5413df054f82a8980523efd712ec55a59e7c3357cf7c/pyparsing-2.2.0.tar.gz should be `https://files.pythonhosted.org/packages/3c/ec/a94f8cf7274ea60b5413df054f82a8980523efd712ec55a59e7c3357cf7c/pyparsing-2.2.0.tar.gz`
  ```

Formula/appscale-tools.rb

@@ -316,12 +316,12 @@ class AppscaleTools < Formula
 
   unless OS.mac?
     resource "secretstorage" do
-      url "https://pypi.python.org/packages/a5/a5/0830cfe34a4cfd0d1c3c8b614ede1edb2aaf999091ac8548dd19cb352e79/SecretStorage-2.3.1.tar.gz"
+      url "https://files.pythonhosted.org/packages/a5/a5/0830cfe34a4cfd0d1c3c8b614ede1edb2aaf999091ac8548dd19cb352e79/SecretStorage-2.3.1.tar.gz"
       sha256 "3af65c87765323e6f64c83575b05393f9e003431959c9395d1791d51497f29b6"
     end
 
     resource "pyparsing" do
-      url "https://pypi.python.org/packages/3c/ec/a94f8cf7274ea60b5413df054f82a8980523efd712ec55a59e7c3357cf7c/pyparsing-2.2.0.tar.gz"
+      url "https://files.pythonhosted.org/packages/3c/ec/a94f8cf7274ea60b5413df054f82a8980523efd712ec55a59e7c3357cf7c/pyparsing-2.2.0.tar.gz"
       sha256 "0832bcf47acd283788593e7a0f542407bd9550a55a8a8435214a1960e04bcb04"
     end
   end