title: using mathfrak in matplotlib label returns Undefined control sequence
body: <p>When I set <code>text.usetex : True</code> in my matplotlibrc and then try to add a text or label with <code>\mathfrak{}</code> I get an error (Undefined control sequence)</p>
  <pre><code>RuntimeError: latex was not able to process the following string:
  b'$\\\\mathfrak{F}$'
  ...
  *geometry* driver: auto-detecting
  *geometry* detected driver: dvips
  ! Undefined control sequence.
  &lt;recently read&gt; \mathfrak 
                            
  l.19 {\rmfamily $\mathfrak
                            {F}$}
  No pages of output.
  </code></pre>
  <p>Other font styles such as <code>\mathcal{}</code> work without issues.</p>
  <p>Another thing that I tried was to set <code>text.usetex : False</code> and then specify that I want to use Computer Modern as the default font but seems like matplotlib is unable to find <code>serif</code> or <code>sans-serif</code> and falls back to DejaVu Sans:</p>
  <pre><code>findfont: Font family ['sans-serif'] not found. Falling back to DejaVu Sans.
  </code></pre>
  <p>Although in this last case, I can use <code>\mathfrak{}</code> without any problems.</p>
  <p>Any ideas how to fix this? Thanks.</p>