title: pyconfig.h missing during "pip install cryptography"
body: <p>I wanna set up scrapy cluster follow this link <a href="http://scrapy-cluster.readthedocs.io/en/latest/topics/introduction/quickstart.html#cluster-quickstart" rel="noreferrer">scrapy-cluster</a>,Everything is ok before I run this command:</p>
  
  <pre><code>pip install -r requirements.txt
  </code></pre>
  
  <p>The requirements.txt looks like:</p>
  
  <pre><code>cffi==1.2.1
  characteristic==14.3.0
  ConcurrentLogHandler&gt;=0.9.1
  cryptography==0.9.1
  ...
  </code></pre>
  
  <p>I guess the above command means to install packages in requirements.txt.But I don't want it to specify the version,So I change it to this:</p>
  
  <pre><code>cat requirements.txt | while read line; do pip install ${line%%[&gt;=]*} --user;done
  </code></pre>
  
  <p>When install cryptography,it gives me the error:</p>
  
  <pre><code>build/temp.linux-x86_64-2.7/_openssl.c:12:24:fatal error:pyconfig.h:No such file or directory 
  #include &lt;pyconfig.h&gt;
  </code></pre>
  
  <p>I don't know how to solved this , I have tried a lot of methods ,but failed. my system is centos 7, and the version of python is 2.7.5(default).
  Besides, Is there any other scrapy frame which is appliable for a large number of urls . Thanks in advance</p>