title: ModuleNotFoundError: No module named 'cartopy' when import SkewT from metpy.plots under Python3
body: <p>When trying to import SkewT into my python3 code on a Mac (Mojave 10.14.6):</p>
  
  <blockquote>
    <p>from metpy.plots import SkewT</p>
  </blockquote>
  
  <p>I get the error:</p>
  
  <blockquote>
    <p>ModuleNotFoundError: No module named 'cartopy'</p>
  </blockquote>
  
  <p>pip3 install cartopy gives the output</p>
  
  <pre><code>Collecting cartopy
    Downloading https://files.pythonhosted.org/packages/e5/92/fe8838fa8158931906dfc4f16c5c1436b3dd2daf83592645b179581403ad/Cartopy-0.17.0.tar.gz (8.9MB)
       |████████████████████████████████| 8.9MB 616kB/s 
    Installing build dependencies ... done
    Getting requirements to build wheel ... error
    ERROR: Complete output from command /usr/local/opt/python/bin/python3.7 /usr/local/lib/python3.7/site-packages/pip/_vendor/pep517/_in_process.py get_requires_for_build_wheel /tmp/tmpj50b1vfe:
    ERROR: setup.py:171: UserWarning: Unable to determine GEOS version. Ensure you have 3.3.3 or later installed, or installation may fail.
      '.'.join(str(v) for v in GEOS_MIN_VERSION), ))
    Proj 4.9.0 must be installed.
    ----------------------------------------
  ERROR: Command "/usr/local/opt/python/bin/python3.7 /usr/local/lib/python3.7/site-packages/pip/_vendor/pep517/_in_process.py get_requires_for_build_wheel /tmp/tmpj50b1vfe" failed with error code 1 in /private/tmp/pip-install-b5cu8485/cartopy
  </code></pre>
  
  <p>To start, I tried to install Proj and geos, but pip3 only lists version 0.1.0 for proj and 0.2.2 for geos. Before I get too far down this rabbit hole, I thought I'd see if anyone else has encountered this problem. Thanks!</p>