title: What is 'extra' in pypi dependency?
body: <p>In <code>requires_dist</code> section of a package's json response from pypi, it is given:</p>
  
  <pre><code>requires_dist : [
      "bcrypt; extra == 'bcrypt'",
      "argon2-cffi (&gt;=16.1.0); extra == 'argon2'"
  ]
  </code></pre>
  
  <p>can anyone make it clear the second statement of each dependency, <code>extra == 'bcrypt'</code> and <code>extra == 'argon2'</code>?</p>