update python 3.6 -> 3.7

Author: vlkong

docs/getting_started_python.html

@@ -84,7 +84,7 @@ <h1>Setting up a Python environment<a class="headerlink" href="#setting-up-a-pyt
 <div class="section" id="system-requirements">
 <h2>System requirements<a class="headerlink" href="#system-requirements" title="Permalink to this headline">&para;</a></h2>
 <ul class="simple">
-<li><strong>Python with version&gt;= 3.6 (including 3.7)</strong> on 64-bit operating systems: Available from  Python.org.<ul>
+<li><strong>Python with version&gt;= 3.7</strong> on 64-bit operating systems: Available from  Python.org.<ul>
 <li>Python version lifespans: <a class="reference external" href="https://docs.python.org/devguide/#status-of-python-branches">here</a></li>
 <li>The Python version you choose must match the CPLEX one if you solve locally.</li>
 </ul>