title: pip3 does not install the target module version
body: <p>Using Python 3.6.9 and pip 9.0.1 on Ubuntu Studio 18.04, I was trying to downgrade the PyQt5 module from version 5.10.1 to 5.9.2.</p>
  
  <p>The installation seems to complete without a problem, but pip3 confirms that <strong>5.10.1</strong> was installed successfully:</p>
  
  <pre><code>pip3 install --no-cache-dir 'PyQt5==5.9.2'
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>Collecting PyQt5==5.9.2
    Downloading https://files.pythonhosted.org/packages/3a/c6/26270f5550f00920045c2f0b222a7d03d7a64382825c68bf0bb1a51d854c/PyQt5-5.9.2-5.9.3-cp35.cp36.cp37-abi3-manylinux1_x86_64.whl (105.3MB)
      100% |████████████████████████████████| 105.3MB 11.0MB/s 
  Collecting sip&lt;4.20,&gt;=4.19.4 (from PyQt5==5.9.2)
    Downloading https://files.pythonhosted.org/packages/8a/ea/d317ce5696dda4df7c156cd60447cda22833b38106c98250eae1451f03ec/sip-4.19.8-cp36-cp36m-manylinux1_x86_64.whl (66kB)
      100% |████████████████████████████████| 71kB 4.2MB/s 
  Installing collected packages: sip, PyQt5
  Successfully installed PyQt5-5.10.1 sip-4.19.8
  </code></pre>
  
  <p>Checking the current version:</p>
  
  <pre><code>pip3 show PyQt5
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>Name: PyQt5
  Version: 5.10.1
  Summary: Python bindings for the Qt cross platform UI and application toolkit
  Home-page: https://www.riverbankcomputing.com/software/pyqt/
  Author: Riverbank Computing Limited
  Author-email: info@riverbankcomputing.com
  License: GPL v3
  Location: /home/stragu/.local/lib/python3.6/site-packages
  Requires: sip
  </code></pre>
  
  <p>I also tried uninstalling and installing it again, but I end up with version 5.10 (i.e. <em>not</em> 5.10.1) installed. It's like it forces version 5.10 as a minimum.</p>
  
  <p>However, when I do the following in a Python3 kernel:</p>
  
  <pre class="lang-py prettyprint-override"><code>from PyQt5.Qt import PYQT_VERSION_STR
  print("PyQt version:", PYQT_VERSION_STR)
  </code></pre>
  
  <p>I find out it is apparently using version 5.9.2!</p>
  
  <p>Any idea what might be going on here?</p>