title: cannot retrieve dataframe in shelve
body: <p>I saved two dataframes onto a <code>shelve</code> database using a previous Pandas version (unsure which one, but likely 0.25.3), and have no problems reading them from the <code>shelve</code> file with Pandas 0.25.3.  However, I cannot read them with the latest Pandas (1.0.0, as printed by pandas.<strong>version</strong>).  I get the following error:</p>
  
  <pre><code>AttributeError: Can't get attribute 'FrozenNDArray' on &lt;module 'pandas.core.indexes.frozen' from 'local/virtualenv.../'
  </code></pre>
  
  <p>It seems the <code>FrozenNDArray</code> class has been removed in 1.0.0, so how can I recover the dataframes?  Some searches point to <code>pandas.read_pickle</code>, but nothing equivalent for <code>shelve</code>.</p>