title: Preserving quotes in numeric keys when writing dictionary to yaml
body: <p>I have yaml files for which <em>some</em> keys have the form 'n_n' for integer n.  Example:</p>
  <pre><code>'1_0':
  &gt;    age: 23
  &gt;
  &gt;    eyes: blue
  </code></pre>
  <p>These are read in fine as python dictionaries, but when written to yaml the &quot;numeric&quot; keys lose their single-quotes and become</p>
  <pre><code>1_0:
  &gt;  age: 23
  &gt;
  &gt;  eyes: blue
  </code></pre>
  <p>And (naturally) on subsequent read/write they become</p>
  <pre><code>10:
  
  &gt;    age: 23
  &gt; 
  &gt;    eyes: blue
  </code></pre>
  <p>I don't want to force <em>every</em> key to be quoted ('age', 'eyes', 'etc').  Is there a solution?</p>