title: pip fails to find any packages from requirements.txt
body: <p>I have a <code>requirements.txt</code> file with following packages listed:</p>
  
  <pre><code>appdirs==1.4.3
  APScheduler==3.3.1
  asn1crypto==0.22.0
  bcrypt==3.1.3
  boto3==1.4.4
  botocore==1.5.72
  </code></pre>
  
  <p>When I do a <code>pip install -q -U -r requirements.txt</code>; I receive following error:</p>
  
  <blockquote>
    <p>Could not find a version that satisfies the requirement appdirs==1.4.3
    (from -r requirements.txt (line 1)) (from versions: )<br>
    No matching distribution found for appdirs==1.4.3 (from -r requirements.txt (line
    1))</p>
  </blockquote>
  
  <p>But, the following works:</p>
  
  <pre><code>$ pip install appdirs
  Collecting appdirs
    Downloading appdirs-1.4.3-py2.py3-none-any.whl
  Installing collected packages: appdirs
  Successfully installed appdirs-1.4.3
  </code></pre>
  
  <p>Once I had manually installed appdirs, I tried running from requirements again:</p>
  
  <pre><code>$ pip install -q -U -r requirements.txt
    Could not find a version that satisfies the requirement APScheduler==3.3.1 (from -r requirements.txt (line 2)) (from versions: )
  No matching distribution found for APScheduler==3.3.1 (from -r requirements.txt (line 2))
  </code></pre>
  
  <p>I tried specifying the exact version for the next command now:</p>
  
  <pre><code>$ pip install APScheduler==3.3.1           
  Collecting APScheduler==3.3.1
    Downloading APScheduler-3.3.1-py2.py3-none-any.whl (56kB)
      100% |████████████████████████████████| 61kB 2.8MB/s 
  Collecting tzlocal&gt;=1.2 (from APScheduler==3.3.1)
    Downloading tzlocal-1.4.tar.gz
  Collecting funcsigs; python_version == "2.7" (from APScheduler==3.3.1)
    Downloading funcsigs-1.0.2-py2.py3-none-any.whl
  Collecting six&gt;=1.4.0 (from APScheduler==3.3.1)
    Downloading six-1.11.0-py2.py3-none-any.whl
  Collecting pytz (from APScheduler==3.3.1)
    Downloading pytz-2017.2-py2.py3-none-any.whl (484kB)
      100% |████████████████████████████████| 491kB 2.2MB/s 
  Requirement already satisfied: setuptools&gt;=0.7 in ./.env/lib/python2.7/site-packages (from APScheduler==3.3.1)
  Collecting futures; python_version == "2.7" (from APScheduler==3.3.1)
    Downloading futures-3.1.1-py2-none-any.whl
  Building wheels for collected packages: tzlocal
    Running setup.py bdist_wheel for tzlocal ... done
    Stored in directory: $HOME/.cache/pip/wheels/dc/d6/f0/84194ccbdd3e2551bdb207e1fc1925efd32956c5767328103b
  Successfully built tzlocal
  Installing collected packages: pytz, tzlocal, funcsigs, six, futures, APScheduler
  Successfully installed APScheduler-3.3.1 funcsigs-1.0.2 futures-3.1.1 pytz-2017.2 six-1.11.0 tzlocal-1.4
  </code></pre>
  
  <p>How is pip not able to find the packages if specifying the <code>requirements</code> file? And why does it keep finding the same packages without it?</p>
  
  <p>PS: Everything is being done inside a <strong><code>virtualenv</code></strong> named <code>.env</code>.</p>
  
  <hr>
  
  <pre><code>$ pip -V
  pip 9.0.1 from $PWD/.env/lib/python2.7/site-packages (python 2.7)
  $ python -V
  Python 2.7.13
  $ which python
  $PWD/.env/bin/python
  </code></pre>
  
  <hr>
  
  <p>Doing a <code>pip --verbose</code> command gives:</p>
  
  <pre><code>Collecting asn1crypto==0.22.0 (from -r requirements.txt (line 3))
    1 location(s) to search for versions of asn1crypto:
    * https://pip.&lt;domain&gt;/asn1crypto/
    Getting page https://pip.&lt;domain&gt;/asn1crypto/
    Looking up "https://pip.&lt;domain&gt;/asn1crypto/" in the cache
    No cache entry available
    Starting new HTTPS connection (1): pip.&lt;domain&gt;
    "GET /asn1crypto/ HTTP/1.1" 303 0
    Status code 303 not in [200, 203, 300, 301]
    Looking up "https://pip.&lt;domain&gt;/simple/asn1crypto/" in the cache
    No cache entry available
    "GET /simple/asn1crypto/ HTTP/1.1" 303 0
    Status code 303 not in [200, 203, 300, 301]
    Starting new HTTP connection (1): pypi.python.org
    "GET /simple/asn1crypto/ HTTP/1.1" 403 16
    Could not fetch URL https://pip.&lt;domain&gt;/asn1crypto/: 403 Client Error: SSL is required for url: http://pypi.python.org/simple/asn1crypto/ - skipping
    Could not find a version that satisfies the requirement asn1crypto==0.22.0 (from -r requirements.txt (line 3)) (from versions: )
  Cleaning up...
  No matching distribution found for asn1crypto==0.22.0 (from -r requirements.txt (line 3))
  </code></pre>
  
  <p>where <code>https://pip.&lt;domain&gt;</code> is our custom hosted <a href="https://pypi.python.org/pypi/pypiserver" rel="nofollow noreferrer">pypi-server layer</a>.</p>