title: Error "Could not find a version that satisfies the requirement flask (from versions: ) No matching distribution found for flask"
body: <p>An exception occurs while installing a package using pip. I tried installing <a href="https://en.wikipedia.org/wiki/NumPy" rel="nofollow noreferrer">NumPy</a>, <a href="https://en.wikipedia.org/wiki/Flask_%28web_framework%29" rel="nofollow noreferrer">Flask</a> and others, but I am getting the below error.</p>
  <pre class="lang-none prettyprint-override"><code>pip install flask
  
  Collecting flask
    Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('&lt;pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x0667CB50&gt;: Failed to establish a new connection: [Errno 11002] getaddrinfo failed')': /simple/flask/
    Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('&lt;pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x0667C190&gt;: Failed to establish a new connection: [Errno 11002] getaddrinfo failed')': /simple/flask/
    Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('&lt;pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x0667C7F0&gt;: Failed to establish a new connection: [Errno 11002] getaddrinfo failed')': /simple/flask/
    Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('&lt;pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x0667C8F0&gt;: Failed to establish a new connection: [Errno 11002] getaddrinfo failed')': /simple/flask/
    Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('&lt;pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x0667C0F0&gt;: Failed to establish a new connection: [Errno 11002] getaddrinfo failed')': /simple/flask/
    Could not find a version that satisfies the requirement flask (from versions: )
  No matching distribution found for flask
  </code></pre>
  <p>How can I resolve it?</p>
  <p>The pip version is 18.1, and the Python version is 3.7.</p>