Python install: update Anaconda URL

The Anaconda corporation has become rather more aggressive about getting users to sign up, as well as pushing their other products.  The current link we use now goes to a "Provide email to download Distribution" screen with an almost invisible "Skip registration" button.  This has baffled at least one learner in my workshop today.

This commit replaces the URL with a link that goes straight to the download page.

_includes/install_instructions/python.html

@@ -13,7 +13,7 @@ <h3>Python</h3>
  research computing, and great for general-purpose programming as
  well. Installing all of its research packages individually can be
  a bit difficult, so we recommend
- <a href="https://www.anaconda.com/products/individual">Anaconda</a>,
+ <a href="https://www.anaconda.com/download/success">Anaconda</a>,
  an all-in-one installer.
  </p>
 
@@ -48,7 +48,7 @@ <h3>Python</h3>
  <div class="tab-content">
  <article role="tabpanel" class="tab-pane active" id="python-windows">
  <ol>
- <li>Open <a href="https://www.anaconda.com/products/individual#download-section">https://www.anaconda.com/products/individual#download-section</a> with your web browser.</li>
+ <li>Open <a href="https://www.anaconda.com/download/success">https://www.anaconda.com/download/success</a> with your web browser.</li>
  <li>Download the Anaconda for Windows installer with Python 3. (If you are not sure which version to choose, you probably want the 64-bit Graphical Installer <em>Anaconda3-...-Windows-x86_64.exe</em>)</li>
  <li>Install Python 3 by running the Anaconda Installer, using all of the defaults for installation <em>except</em> make sure to check <strong>Add Anaconda to my PATH environment variable</strong>.</li>
  </ol>
@@ -61,7 +61,7 @@ <h4>Video Tutorial</h4>
  </article>
  <article role="tabpanel" class="tab-pane" id="python-macos">
  <ol>
- <li>Open <a href="https://www.anaconda.com/products/individual#download-section">https://www.anaconda.com/products/individual#download-section</a> with your web browser.</li>
+ <li>Open <a href="https://www.anaconda.com/download/success">https://www.anaconda.com/download/success</a> with your web browser.</li>
  <li>Download the Anaconda Installer with Python 3 for macOS (you can either use the Graphical or the Command Line Installer).</li>
  <li>Install Python 3 by running the Anaconda Installer using all of the defaults for installation.</li>
  </ol>
@@ -74,7 +74,7 @@ <h4>Video Tutorial</h4>
  </article>
  <article role="tabpanel" class="tab-pane" id="python-linux">
  <ol>
- <li>Open <a href="https://www.anaconda.com/products/individual#download-section">https://www.anaconda.com/products/individual#download-section</a> with your web browser.</li>
+ <li>Open <a href="https://www.anaconda.com/download/success">https://www.anaconda.com/download/success</a> with your web browser.</li>
  <li>Download the Anaconda Installer with Python 3 for Linux.<br>
  (The installation requires using the shell. If you aren't
  comfortable doing the installation yourself