text: Title: Cannot install cartopy in Scientific Linux
  Body: ### Description
  <!-- Please provide a general introduction to the issue/proposal. -->
  Cartopy cannot be installed on Scientific Linux because the newest available version of `proj` is 4.8.0 and cartopy requires 4.9.0. I'm not sure why `setup.py` checks the system-level versions of `proj` and `geos` instead of the versions in the current virtualenv though...
  
  <!--
  If you are reporting a bug, attach the *entire* traceback from Python.
  
  If you are proposing an enhancement/new feature, provide links to related articles, reference examples, etc.
  
  If you are asking a question, please ask on StackOverflow and use the cartopy tag. All cartopy
  questions on StackOverflow can be found at https://stackoverflow.com/questions/tagged/cartopy
  -->
  
  #### Code to reproduce
  
  ```
  pip install cartopy
  ```
  
  #### Traceback 
  
  ```
  Collecting cartopy
    Using cached Cartopy-0.17.0.tar.gz (8.9 MB)
    Installing build dependencies ... done
    Getting requirements to build wheel ... error
    ERROR: Command errored out with exit status 1:
     command: /home/jba/venv/gdal3/bin/python3 /home/jba/venv/gdal3/lib64/python3.6/site-packages/pip/_vendor/pep517/_in_process.py get_requires_for_build_wheel /tmp/tmp312hzzaa
         cwd: /tmp/pip-install-u9agv4jo/cartopy
    Complete output (1 lines):
    Proj version 4.8.0 is installed, but cartopy requires at least version 4.9.0.
    ----------------------------------------
  ERROR: Command errored out with exit status 1: /home/jba/venv/gdal3/bin/python3 /home/jba/venv/gdal3/lib64/python3.6/site-packages/pip/_vendor/pep517/_in_process.py get_requires_for_build_wheel /tmp/tmp312hzzaa Check the logs for full command output.
  
  ```
  
  <details>
   <summary>Full environment definition</summary>
  
  <!-- fill in the following information as appropriate -->
  
  ### Operating system
  $ cat /proc/version 
  Linux version 3.10.0-1062.18.1.el7.x86_64 (mockbuild@sl7-uefisign.fnal.gov) (gcc version 4.8.5 20150623 (Red Hat 4.8.5-39) (GCC) ) #1 SMP Tue Mar 17 10:44:42 CDT 2020
  
  $ cat /etc/*-release
  NAME="Scientific Linux"
  VERSION="7.4 (Nitrogen)"
  ID="scientific"
  ID_LIKE="rhel centos fedora"
  VERSION_ID="7.4"
  PRETTY_NAME="Scientific Linux 7.4 (Nitrogen)"
  ANSI_COLOR="0;31"
  CPE_NAME="cpe:/o:scientificlinux:scientificlinux:7.4:GA"
  HOME_URL="http://www.scientificlinux.org//"
  BUG_REPORT_URL="mailto:scientific-linux-devel@listserv.fnal.gov"
  
  REDHAT_BUGZILLA_PRODUCT="Scientific Linux 7"
  REDHAT_BUGZILLA_PRODUCT_VERSION=7.4
  REDHAT_SUPPORT_PRODUCT="Scientific Linux"
  REDHAT_SUPPORT_PRODUCT_VERSION="7.4"
  Scientific Linux release 7.4 (Nitrogen)
  Scientific Linux release 7.4 (Nitrogen)
  Scientific Linux release 7.4 (Nitrogen)
  
  ### Cartopy version
  0.17.0
  
  ### conda list
  
  ```
  n/a
  ```
  
  ### pip list
  
  ```
  Package               Version Location              
  --------------------- ------- ----------------------
  arrow                 0.15.5  
  attrs                 19.3.0  
  backcall              0.1.0   
  chardet               3.0.4   
  click                 7.1.1   
  cloudpickle           1.3.0   
  colorama              0.4.3   
  coverage              5.0.3   
  cycler                0.10.0  
  Cython                0.29.15 
  dask                  2.12.0  
  decorator             4.4.2   
  entrypoints           0.3     
  farmlib               1.5.9   /home/jba/work/farmlib
  flake8                3.7.9   
  flake8-builtins       1.4.2   
  flake8-commas         2.0.0   
  flake8-comprehensions 3.2.2   
  flake8-docstrings     1.5.0   
  flake8-logging-format 0.6.0   
  flake8-polyfill       1.0.2   
  flake8-print          3.1.4   
  flake8-string-format  0.3.0   
  future                0.18.2  
  fuzzywuzzy            0.17.0  
  GDAL                  3.0.4   
  importlib-metadata    1.5.0   
  ipython               7.1.1   
  ipython-genutils      0.2.0   
  jbafarm               1.7.3   /home/jba/work/farmmap
  jedi                  0.16.0  
  jsonschema            3.2.0   
  kiwisolver            1.1.0   
  litmus                1.3.4   
  matplotlib            3.2.0   
  mccabe                0.6.1   
  mock                  4.0.2   
  networkx              2.4     
  nose                  1.3.7   
  nosexcover            1.0.11  
  numpy                 1.18.1  
  packaging             20.3    
  pandas                0.25.0  
  parso                 0.6.2   
  pep8-naming           0.9.1   
  pexpect               4.8.0   
  pickleshare           0.7.5   
  Pillow                7.0.0   
  pip                   20.0.2  
  proj                  0.1.0   
  prompt-toolkit        2.0.10  
  psycopg2-binary       2.8.4   
  ptyprocess            0.6.0   
  pycodestyle           2.5.0   
  pydocstyle            3.0.0   
  pyflakes              2.1.1   
  Pygments              2.6.1   
  pyparsing             2.4.6   
  pyproj                2.1.3   
  pyrsistent            0.15.7  
  python-dateutil       2.8.1   
  python-Levenshtein    0.12.0  
  pytz                  2019.3  
  PyWavelets            1.0.3   
  Rtree                 0.9.4   
  scikit-image          0.14.5  
  scipy                 1.2.1   
  setuptools            39.2.0  
  Shapely               1.7.0   
  simplejson            3.17.0  
  six                   1.14.0  
  snowballstemmer       2.0.0   
  toolz                 0.10.0  
  tqdm                  4.43.0  
  traitlets             4.3.3   
  wcwidth               0.1.8   
  wheel                 0.34.2  
  zipp                  3.1.0   
  
  ```
  
  </details>
  